Merging r282182:

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r282182 | nemanja.i.ibm | 2016-09-22 12:06:38 -0700 (Thu, 22 Sep 2016) | 6 lines

[PowerPC] Sign extend sub-word values for atomic comparisons

Atomic comparison instructions use the sub-word load instruction on
Power8 and up but the value is not sign extended prior to the signed word
compare instruction. This patch adds that sign extension.

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llvm-svn: 287811