[ARM] Don't abort on variable-idx extractelt in ReconstructShuffle.

[ARM] Don't abort on variable-idx extractelt in ReconstructShuffle.

The code introduced in r244314 assumed that EXTRACT_VECTOR_ELT only
takes constant indices, but it does accept variables.
Bail out for those: we can't use them, as the shuffles we want to
reconstruct do require constant masks.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@246594 91177308-0d34-0410-b5e6-96231b3b80d8