[GlobalISel] Fix legalizer artifact combiner from crashing with invalid dead instructions.

The artifact combiners push instructions which have been marked for deletion
onto an list for the legalizer to deal with on return. However, for trunc(ext)
combines the combiner routine recursively calls itself. When it does this the
dead instructions list may not be empty, and the other combiners don't expect
to be dealing with essentially invalid MIR (multiple vreg defs etc).

This change fixes it by ensuring that the dead instructions are processed on
entry into tryCombineInstruction.

As a result, this fix exposed a few places in tests where G_TRUNC instructions
were not being deleted even though they were dead.

Differential Revision: https://reviews.llvm.org/D59892

llvm-svn: 357101