[X86] combineCMP - fold cmpEQ/NE(TRUNC(X),0) -> cmpEQ/NE(X,0)

If we are truncating from a i32 source before comparing the result against zero, then see if we can directly compare the source value against zero.

If the upper (truncated) bits are known to be zero then we can compare against that, hopefully increasing the chances of us folding the compare into a EFLAG result of the source's operation.

Fixes PR49028.

Differential Revision: https://reviews.llvm.org/D100491