AMDGPU: Add 24-bit mul intrinsics

AMDGPU: Add 24-bit mul intrinsics

Insert these during codegenprepare.

This works around a DAG issue where generic combines eliminate the and
asserting the high bits are zero, which then exposes an unknown read
source to the mul combine. It doesn't worth the hassle of trying to
insert an AssertZext or something to try to deal with it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@366094 91177308-0d34-0410-b5e6-96231b3b80d8