Merging r294129:

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r294129 | gberry | 2017-02-05 10:28:14 -0800 (Sun, 05 Feb 2017) | 16 lines

[SelectionDAG] In InstrEmitter, handle EXTRACT_SUBREG of a physical register.

Summary:
Without this change, the getVR() call would hit an assert since it was
being passed a physical register.

Update the AArch64/ldst-opt.ll test with a case that triggers this
behavior by adding a run with strict-align, which causes an unaligned
STR XZR instruction to be split into byte stores, creating an
EXTRACT_SUBREG of XZR that triggers the original problem.

Reviewers: bogner, qcolombet, MatzeB, atrick

Subscribers: aemerson, mcrosier, llvm-commits

Differential Revision: https://reviews.llvm.org/D29495
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llvm-svn: 295250