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[ARM] Don't reserve R12 on Thumb1 as an emergency spill slot.

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[ARM] Don't reserve R12 on Thumb1 as an emergency spill slot.

The current implementation of ThumbRegisterInfo::saveScavengerRegister
is bad for two reasons: one, it's buggy, and two, it blocks using R12
for other optimizations. So this patch gets rid of it, and adds the
necessary support for using an ordinary emergency spill slot on Thumb1.

(Specifically, I think saveScavengerRegister was broken by r305625, and
nobody noticed for two years because the codepath is almost never used.
The new code will also probably not be used much, but it now has better
tests, and if we fail to emit a necessary emergency spill slot we get a
reasonable error message instead of a miscompile.)

A rough outline of the changes in the patch:

  1. Gets rid of ThumbRegisterInfo::saveScavengerRegister.
  2. Modifies ARMFrameLowering::determineCalleeSaves to allocate an

emergency spill slot for Thumb1.

  1. Implements useFPForScavengingIndex, so the emergency spill slot isn't

placed at a negative offset from FP on Thumb1.

  1. Modifies the heuristics for allocating an emergency spill slot to

support Thumb1. This includes fixing ExtraCSSpill so we don't try to
use "lr" as a substitute for allocating an emergency spill slot.

  1. Allocates a base pointer in more cases, so the emergency spill slot

is always accessible.

  1. Modifies ARMFrameLowering::ResolveFrameIndexReference to compute the

right offset in the new cases where we're forcing a base pointer.

  1. Ensures we never generate a load or store with an offset outside of

its frame object. This makes the heuristics more straightforward.

  1. Changes Thumb1 prologue and epilogue emission so it never uses

register scavenging.

Some of the changes to the emergency spill slot heuristics in
determineCalleeSaves affect ARM/Thumb2; hopefully, they should allow
the compiler to avoid allocating an emergency spill slot in cases
where it isn't necessary. The rest of the changes should only affect
Thumb1.

Differential Revision: https://reviews.llvm.org/D63677

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