[AArch64] Fix assertion failure caused by an invalid comparison between APInt values.

APInt only knows how to compare values with the same BitWidth and asserts
in all other cases.

With this fix, function PerformORCombine does not use the APInt equality
operator if the APInt values returned by 'isConstantSplat' differ in BitWidth.
In that case they are different and no comparison is needed.

llvm-svn: 199119