[SPIR-V] Fold constant instructions when there's `-Gis` flag

https://godbolt.org/z/95WvzPfYz

Description
Context

We should fold certain instructions even when there's -Gis flag, such when all parameters are known constants.

Steps to Reproduce

// RUN: %dxc -spirv -fspv-target-env=vulkan1.3 -enable-16bit-types -Gis -HV 202x -T cs_6_5 %s

RWStructuredBuffer<float4> Out1 : register(u5);

[numthreads(1,1,1)]
void main() {
    // both of these are hitting the assert
    Out1[0] = pow(float4(2, 4, 1.5, 8), float4(3, 0.5, 2, 0.33333333));
}

is said to produce the error

# .---command stderr------------
# | Internal compiler error: LLVM Assert
# `-----------------------------
# error: command failed with exit status: 0xe0000001

Error: assert((!HasStatus(inst) || Status(inst) != SSAPropagator::kNotInteresting) && "Unsettled value")
File:
/home/kaitlinpeng/DirectXShaderCompiler/external/SPIRV-Tools/source/opt/propagator.cpp(265)
Func:   operator()
Illegal instruction (core dumped)

Actual Behavior
The instructions shouldn't throw errors and properly fold even when there's -Gis flag.