[hlsl-out] Fix countOneBits and reverseBits for signed integers

[hasali19]

This PR fixes usage of the HLSL countbits and reversebits functions with signed integers, by adding the necessary asuint and asint conversions.

Fixes #1904

[teoxoy]

Looks good! Could you add HLSL to this list and generate the snapshot for it?

naga/tests/snapshots.rs

Lines 418 to 419 in 60ae549

	"bits",
	Targets::SPIRV | Targets::METAL | Targets::GLSL | Targets::WGSL,

[teoxoy]

Could we name this NoIntOverload or MissingIntOverload? What do you think?

[hasali19]

I was trying to think of a good name, that sounds much better!

[hasali19]

Looks good! Could you add HLSL to this list and generate the snapshot for it?

naga/tests/snapshots.rs

Lines 418 to 419 in 60ae549

	"bits",
	Targets::SPIRV | Targets::METAL | Targets::GLSL | Targets::WGSL,

I noticed that previously and attempted to add it, but it looks like there are some other functions which aren't implemented yet causing it to fail:

thread 'convert_wgsl' panicked at 'HLSL write failed: Unimplemented("write_expr_math Pack4x8snorm")', tests/snapshots.rs:313:54
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[teoxoy]

Alright, we can enable the test later then. Thanks!