Minor refactoring regarding Cast::isNoopCast(), NFC

Summary:
FastISel::hasTrivialKill() was the only user of the "IntPtrTy" version of
Cast::isNoopCast(). According to review comments in D37894 we could instead
use the "DataLayout" version of the method, and thus get rid of the
"IntPtrTy" versions of isNoopCast() completely.

With the above done, the remaining isNoopCast() could then be simplified
a bit more.

Reviewers: arsenm

Reviewed By: arsenm

Subscribers: wdng, llvm-commits

Differential Revision: https://reviews.llvm.org/D38497

llvm-svn: 314969