[AMDGPU] Remove MUL_LOHI_U24/MUL_LOHI_I24

These were introduced in r279902 on the grounds that using separate
MUL_U24/MUL_I24 and MULHI_U24/MULHI_I24 nodes would introduce multiple
uses of the operands, which would prevent SimplifyDemandedBits from
simplifying the operands.

This has since been fixed by D24672 "AMDGPU/SI: Use new SimplifyDemandedBits helper for multi-use operations"

No functional change intended. At least it has no effect on lit tests.

Differential Revision: https://reviews.llvm.org/D89706