[AMDGPU] Update WMMA intrinsics with explicit f16 types

Update intrinsics to use n x f16 and n x i16 instead
of 32-bit types. This may avoid the need for a bitcast
and is probably less confusing.

Depends on making v16f16 and v16i16 types legal.

Reviewed By: rampitec

Differential Revision: https://reviews.llvm.org/D128951