[HLSL] static constants are not fully optimized

[s-perron]

The example below will generate a call to an intrinsic @llvm.invariant.start.p0 it indicate that the variable will not change after it is initialized. However, this seems to hinder optimizations. The store to the variable is still present after optimizations. See https://godbolt.org/z/fT39qoafr.

RWBuffer<int> b : register(u0);

int foo() { return 20; }

static const int c = foo();

[numthreads(1,1,1)]
void main() {
    b[0] = c;
}