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

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@341642 91177308-0d34-0410-b5e6-96231b3b80d8