[AMDGPU] Don't transform illegal intrinsics to V_ILLEGAL

This reverts parts of D123693. The functionality of allowing unsupported
intrinsics to select has been superseded by D139000 "Remove function
with incompatible features".

Retain assembler/disassembler support for v_illegal on GFX10+ only,
where it is documented.

Differential Revision: https://reviews.llvm.org/D148127