[SPIR-V] Add WaveGetLaneIndex() intrinsic support (#85979)

Add support to generate valid SPIR-V for the WaveGetLaneIndex() HLSL
builtin.

To implement this, I had to fix a few small issues in the backend, like
the i8* pointer type being emitted, even if we have the type information
elsewhere.

Signed-off-by: Nathan Gauër <brioche@google.com>