Add some missing checks in TwoAddressInstructionPass::CoalesceExtSubRegs.

Add some missing checks in TwoAddressInstructionPass::CoalesceExtSubRegs.
Check that all the instructions are in the same basic block, that the
EXTRACT_SUBREGs write to the same subregs that are being extracted, and that
the source and destination registers are in the same regclass.  Some of
these constraints can be relaxed with a bit more work.  Jakob suggested
that the loop that checks for subregs when NewSubIdx != 0 should use the
"nodbg" iterator, so I made that change here, too.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@105437 91177308-0d34-0410-b5e6-96231b3b80d8