[AMDGPU] Fix insert of SIPreAllocateWWMRegs in FastRegAlloc

SIPreAllocateWWMRegs was being inserted after RegisterCoalescer
but this pass does not exist during FastAlloc so pre-allocation
pass was never being run.
Insert pre-allocation after TwoAddressInstructionPass instead.

Reviewed By: rampitec

Differential Revision: https://reviews.llvm.org/D90236