[AMDGPU] Fix 24-bit mul intrinsic generation for > 32-bit result.

The 24-bit mul intrinsics yields the low-order 32 bits. We should only
do the transformation if the operands are known to be not wider than 24
bits and the result is known to be not wider than 32 bits.

Differential Revision: https://reviews.llvm.org/D111523