Merging r341642:

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r341642 | tnorthover | 2018-09-07 11:21:25 +0200 (Fri, 07 Sep 2018) | 8 lines

ARM: fix Thumb2 CodeGen for ldrex with folded frame-index.

Because t2LDREX (& t2STREX) were marked as AddrModeNone, but did allow a
FrameIndex operand, rewriteT2FrameIndex asserted. This gives them a
proper addressing-mode and tells the rewriter about it so that encodable
offsets are exploited and others are rejected.

Should fix PR38828.
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git-svn-id: https://llvm.org/svn/llvm-project/llvm/branches/release_70@341783 91177308-0d34-0410-b5e6-96231b3b80d8