[GlobalISel][AArch64] Gardening: Factor out code to find lane indices

Some more refactoring for G_INSERT_VECTOR_ELT.

Factor out the code used to find a lane index from `selectExtractElt`. Put it
into a more general-purpose `getConstantValueForReg` function.

This will be shared with the code for G_INSERT_VECTOR_ELT.

Differential Revision: https://reviews.llvm.org/D59324

llvm-svn: 356101