[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

llvm-svn: 358977