[SPIR-V] fatal error: generated SPIR-V is invalid: GLSL.std.450 FindUMsb: expected all operands to have the same bit width as Result

[jayfoad]

With a dxc that I have just built from the main branch I get:

$ cat z.hlsl 
[numthreads(1, 1, 1)]
void entrypoint()
{
  uint64_t a = 0;
  a = firstbithigh(a);
}

$ build/bin/dxc -fspv-target-env=vulkan1.1 -spirv -E entrypoint -T cs_6_2 -Od z.hlsl
fatal error: generated SPIR-V is invalid: GLSL.std.450 FindUMsb: expected all operands to have the same bit width as Result Type
  %15 = OpExtInst %uint %1 FindUMsb %13

[s-perron]

Duplicate of #4702.