[AMDGPU] Don't shrink VOP3 instructions pre-RA on GFX10+

In GFX10, there is no advantage to shrinking these instructions pre-RA,
so this just saves a bit of work.

In GFX11 there is an advantage to *not* shrinking them pre-RA, because
the register classes for 16-bit operands are less restrictive in the
VOP3 form than in the shrunk form. This patch is a prerequisite for
actually setting up those register classes correctly for 16-bit vs
non-16-bit operands.

Differential Revision: https://reviews.llvm.org/D133769