[DirectX] Make dx.RawBuffer an op that can't be replaced (#154620)
fixes #152348
SimplifyCFG collapses raw buffer store from a if\else load into a
select.
This change prevents the TargetExtType dx.Rawbuffer from being replace
thus preserving the if\else blocks.
A further change was needed to eliminate the phi node before we process
Intrinsic::dx_resource_getpointer in DXILResourceAccess.cpp