AArch64: don't try to handle [SU]MUL_LOHI nodes

We should set them to expand for now since there are no patterns
dealing with them. Actually, there are no instructions either so I
doubt they'll ever be acceptable.
ARM: correctly determine final tBX_LR in Thumb1 functions

The changes caused by folding an sp-adjustment into a "pop" previously
disrupted the forward search for the final real instruction in a
terminating block. This switches to a backward search (skipping debug
instrs).

This fixes PR18399.

Patch by Zhaoshi.