[ARM] Add ARMISD::VLD1DUP to match vld1_dup more consistently.

Currently, there are substantial problems forming vld1_dup even if the
VDUP survives legalization. The lack of an actual node
leads to terrible results: not only can we not form post-increment vld1_dup
instructions, but we form scalar pre-increment and post-increment
loads which force the loaded value into a GPR. This patch fixes that
by combining the vdup+load into an ARMISD node before DAGCombine
messes it up.

Also includes a crash fix for vld2_dup (see testcase @vld2dupi8_postinc_variable).

Differential Revision: https://reviews.llvm.org/D27694

llvm-svn: 289703