AMDGPU: Figure out private memory regs after lowering

Introduce pseudo-registers for registers needed for stack
access, which are replaced during finalizeLowering.
Note these pseudo-registers are currently only used for the
used register location, and not for determining their
input argument register.

This is better because it avoids the need to try to predict
whether a call will be emitted from the IR, and also
detects stack objects introduced by legalization.

Test changes are from the HasStackObjects check being more
accurate since stack objects introduced during legalization
are now known.

llvm-svn: 308325