Add intrinsics for the new FP conversions introduced by the 2024 dpISA (#407)

FEAT_FPRCVT adds 4 new variants for each FCVTAS, FCVTAU, FCVTMS, FCVTMU,
FCVTNS, FCVTNU, FCVTPS, FCVTPU, FCVTZS, and FCVTZU instruction.
1) Half Precision to 32-bit
2) Half Precision to 64-bit
3) Single Precision to 64-bit
4) Double Precision to 32-bit

For the Single Precision to 64-bit and Double Precision to 32-bit
variants, this patch adds two new intrinsics, that reduce to
- Single Precision to 64-bit : <INST> Dd,Sn
- Double Precision to 32-bit : <INST> Sd,Dn

The intrinsics for conversions from Half Precision are already defined.
However they are documented as reducing to the incorrect instruction
format; <INST> Hd,Hn, so this patch fixes them to be
- Half Precision to 32-bit   : <INST> Sd,Hn
- Half Precision to 64-bit   : <INST> Dd,Hn

Author: amilendra

main/acle.md

@@ -483,6 +483,7 @@ Armv8.4-A [[ARMARMv84]](#ARMARMv84). Support is added for the Dot Product intrin
   support for SME2.2 (FEAT_SME2p2).
 * Added [**Alpha**](#current-status-and-anticipated-changes) support
   for FEAT_SVE_AES2, FEAT_SSVE_AES intrinsics.
+* Added support for FEAT_FPRCVT intrinsics and `__ARM_FEATURE_FPRCVT`.
 
 ### References
 
@@ -2248,6 +2249,13 @@ ACLE intrinsics are available. This implies that `__ARM_FEATURE_SM4` and
 floating-point absolute minimum and maximum instructions (FEAT_FAMINMAX)
 and if the associated ACLE intrinsics are available.
 
+### FPRCVT extension
+
+`__ARM_FEATURE_FPRCVT` is defined to `1` if there is hardware
+support for floating-point to/from integer conversion instructions
+with only scalar SIMD&FP register operands and results, and with
+different input and output register sizes.
+
 ### Lookup table extensions
 
 `__ARM_FEATURE_LUT` is defined to 1 if there is hardware support for
@@ -2651,6 +2659,7 @@ be found in [[BA]](#BA).
 | [`__ARM_FEATURE_FP8DOT2`](#modal-8-bit-floating-point-extensions)                                                                                       | Modal 8-bit floating-point extensions                                                              | 1           |
 | [`__ARM_FEATURE_FP8DOT4`](#modal-8-bit-floating-point-extensions)                                                                                       | Modal 8-bit floating-point extensions                                                              | 1           |
 | [`__ARM_FEATURE_FP8FMA`](#modal-8-bit-floating-point-extensions)                                                                                        | Modal 8-bit floating-point extensions                                                              | 1           |
+| [`__ARM_FEATURE_FPRCVT`](#fprcvt-extension)                                                                                                             | FPRCVT extension                                                                                   | 1           |
 | [`__ARM_FEATURE_FRINT`](#availability-of-armv8.5-a-floating-point-rounding-intrinsics)                                                                  | Floating-point rounding extension (Arm v8.5-A)                                                     | 1           |
 | [`__ARM_FEATURE_GCS`](#guarded-control-stack)                                                                                                           | Guarded Control Stack                                                                              | 1           |
 | [`__ARM_FEATURE_GCS_DEFAULT`](#guarded-control-stack)                                                                                                   | Guarded Control Stack protection can be enabled                                                    | 1           |