[X86] Move stack folding test for MULX to a MIR test. Add a MULX32 case as well

A future patch may stop using MULX by default so use MIR to ensure we're always testing MULX.

Add the 32-bit case that we couldn't do in the 64-bit mode IR test due to it being promoted to a 64-bit mul.

llvm-svn: 348972