MIR: Preserve incoming frame index numbers

Don't skip incrementing the frame index number
if the object is dead. Instructions can still be
referencing the old frame index number, and this
doesn't attempt to remap those. The resulting
MIR then fails to load because the use instructions
use a higher frame index number than recorded
list of stack objects.

I'm not sure it's possible to craft a testcase
with the existing set of passes. It requires
selectively marking some stack objects
dead in an essentially random order.
StackSlotColoring condenses towards
the low indexes. This avoids a regression in a
future AMDGPU commit when some frame indexes
are lowered separately from PEI.

llvm-svn: 354688