Add instruction encodings / disassembly support for l6r instructions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173288 91177308-0d34-0410-b5e6-96231b3b80d8
ConstantFolding: Evaluate GEP indices in the index type.

This fixes some edge cases that we would get wrong with uint64_ts.
PR14986.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173289 91177308-0d34-0410-b5e6-96231b3b80d8