`Winch internal error: Unsupported immediate` on a few Sightglass benchmarks

[fitzgen]

To repro, clone Sightglass and run this command in the Wasmtime repo:

$ cargo run -- compile -C compiler=winch -- path/to/sightglass/benchmarks/meshoptimizer/benchmark.wasm
Error: failed to compile: wasm[0]::function[14]

Caused by:
    Compilation error: Winch internal error: Unsupported immediate

It also fails on a few of the libsodium benchmarks, maybe others as well.

+cc @saulecabrera

[saulecabrera]

Thanks Nick; I will take a look.