[AMDGPU] Add S_MOV_B64_IMM_PSEUDO for wide constants

This is to allow 64 bit constant rematerialization. If a constant
is split into two separate moves initializing sub0 and sub1 like
now RA cannot rematerizalize a 64 bit register.

This gives 10-20% uplift in a set of huge apps heavily using double
precession math.

Fixes: SWDEV-292645

Differential Revision: https://reviews.llvm.org/D104874