[ARM] Look through concat when lowering in-place shuffles (VZIP, ..)

Currently, we canonicalize shuffles that produce a result larger than
their operands with:
  shuffle(concat(v1, undef), concat(v2, undef))
->
  shuffle(concat(v1, v2), undef)

because we can access quad vectors (see PerformVECTOR_SHUFFLECombine).

This is useful in the general case, but there are special cases where
native shuffles produce larger results: the two-result ops.

We can look through the concat when lowering them:
  shuffle(concat(v1, v2), undef)
->
  concat(VZIP(v1, v2):0, :1)

This lets us generate the native shuffles instead of scalarizing to
dozens of VMOVs.

Differential Revision: http://reviews.llvm.org/D10424

llvm-svn: 240118