[GlobalISel][AArch64] Use getConstantVRegValWithLookThrough for extracts

getConstantVRegValWithLookThrough does the same thing as the
getConstantValueForReg function, and has more visibility across GISel. Plus, it
supports looking through G_TRUNC, G_SEXT, and G_ZEXT. So, we get better code
reuse and more functionality for free by using it.

Add some test cases to select-extract-vector-elt.mir to show that we can now
look through those instructions.

llvm-svn: 359351