[ARM][ARMCGP] Remove unecessary zexts and truncs

r345840 slightly changed the way promotion happens which could
result in zext and truncs having the same source and destination
types. This fixes that issue.

We can now also remove the zext and trunc in the following case:
(zext (trunc (promoted op)), i32)

This means that we can no longer treat a value, that is only used by
a sink, to be safe to promote.

I've also added in some extra asserts and replaced a cast for a
dyn_cast.

Differential Revision: https://reviews.llvm.org/D54032

llvm-svn: 346125