Revert: r343521 and r343541: [globalisel] Add a combiner helpers for extending...

Revert: r343521 and r343541: [globalisel] Add a combiner helpers for extending loads and use them in a pre-legalize combiner for AArch64

There's a strange assertion on two of the Green Dragon bots that goes away when
this is reverted. The assertion is in RegBankAlloc and if it is this commit then
-verify-machine-instrs should have caught it earlier in the pipeline.

llvm-svn: 343546