[ARM][CGP] Check trunc type before replacing

[ARM][CGP] Check trunc type before replacing

In the last stage of type promotion, we replace any zext that uses a
new trunc with the operand of the trunc. This is okay when we only
allowed one type to be optimised, but now its the case that the trunc
maybe needed to produce a more narrow type than the one we were
optimising for. So we need to check this before doing the replacement.

Differential Revision: https://reviews.llvm.org/D57041

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@351935 91177308-0d34-0410-b5e6-96231b3b80d8