[InstCombine] canonicalize insertelement of scalar constant ahead of insertelement of variable

insertelement (insertelement X, Y, IdxC1), ScalarC, IdxC2 -->
insertelement (insertelement X, ScalarC, IdxC2), Y, IdxC1

As noted in the code comment and seen in the test changes, the motivation is that by pulling
constant insertion up, we may be able to constant fold some insertelement instructions.

Differential Revision: https://reviews.llvm.org/D31196

llvm-svn: 298520