[AMDGPU] Stop mulhi from doing 24 bit mul for uniform values

Added support to check if architecture supports s_mulhi which is used as part of
the decision whether or not to use valu 24 bit mul (if the mulhi gets
transformed to a valu op anyway, then may as well use it).

This is an extension of the work in D97063

Differential Revision: https://reviews.llvm.org/D103321

Change-Id: I80b1323de640a52623d69ac005a97d06a5d42a14