[mlir][AMDGPU] Add packed 8-bit float conversion ops and lowering

Define operations that wrap the gfx940's new operations for converting
between f32 and registers containing packed sets of four 8-bit floats.

Define rocdl operations for the intrinsics and an AMDGPU dialect
wrapper around them (to account for the fact that MLIR distinguishes
the two float formats at the type level but that the LLVM IR does
not).

Define an ArithToAMDGPU pass, meant to run before conversion to LLVM,
that replaces relevant calls to arith.extf and arith.truncf with the
packed operations in the AMDGPU dialect. Note that the conversion
currently only handles scalars and vectors of rank <= 1, as we do not
have a usecase for multi-dimensional vector support right now.

Reviewed By: jsjodin

Differential Revision: https://reviews.llvm.org/D152457