[ARM] Update check for CBZ in Ifcvt

[ARM] Update check for CBZ in Ifcvt

The check for creating CBZ in constant island pass recently obtained the
ability to search backwards to find a Cmp instruction. The code in IfCvt should
mirror this to allow more conversions to the smaller form. The common code has
been pulled out into a separate function to be shared between the two places.

Differential Revision: https://reviews.llvm.org/D60090

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@358977 91177308-0d34-0410-b5e6-96231b3b80d8