[AMDGPU] GFX11: rename VALU pknorm instructions to pk_norm

GFX11 renames:
v_cvt_pknorm_i16_f32 to v_cvt_pk_norm_i16_f32
v_cvt_pknorm_u16_f32 to v_cvt_pk_norm_u16_f32

Accept the old name as an alias.

Fixes https://github.com/llvm/llvm-project/issues/60334

Differential Revision: https://reviews.llvm.org/D143266