[GlobalISel][AArch64] Add selection support for G_EXTRACT_VECTOR_ELT

This adds instruction selection support for G_EXTRACT_VECTOR_ELT for cases
where the index is defined by a G_CONSTANT.

It also factos out the lane copy opcode selection part into its own function,
`getLaneCopyOpcode`. This is used by both `selectUnmergeValues` and
`selectExtractElt`.

Differential Revision: https://reviews.llvm.org/D58469

llvm-svn: 355344