hexagon: Add compile-time validation for scalar immediate operands

Improve the scalar intrinsic generator with three changes:

- Immediate operands now use const generics with static assertions
  (static_assert_uimm_bits!/static_assert_simm_bits!) so out-of-range
  values and runtime variables are rejected at compile time. Unsigned
  immediates use u32, signed use i32, matching the LoongArch convention.
  rustc_legacy_const_generics preserves positional call syntax:
  Q6_R_add_RI(rs, 42) rather than Q6_R_add_RI::<42>(rs).

- Fix missing assert_instr on void-returning instructions (dmlink,
  dmresume, dmstart) whose assembly syntax has no assignment operator.
  extract_instr_name now falls back to extract_leading_mnemonic for
  function-call-style syntax.

- Hoist regex compilation out of parse_prototype (called ~900x) into
  parse_header where they are compiled once and passed by reference.

Author: androm3da