[GlobalISel] Fix legalizer trying to process a deleted instruction.

In some cases an instruction is deleted from the block during combining,
however it can still exist in the legalizer worklist.

This change modifies the combiner routines to add the given MI to the
dead instruction list rather than trying to remove it from the block
themselves. The responsibility is then on the caller to delete the instruction
from the block and any worklists.

Differential Revision: https://reviews.llvm.org/D38622

llvm-svn: 315092