[AArch64] Fix performPostLD1Combine to check for constant lane index.

Summary:
performPostLD1Combine in AArch64ISelLowering looks for vector
insert_vector_elt of a loaded value which it can optimize into a single
LD1LANE instruction.  The code checking for the pattern was not checking
if the lane index was a constant which could cause two problems:

- an assert when lowering the LD1LANE ISD node since it assumes an
  constant operand

- an assert in isel if the lane index value depends on the
  post-incremented base register

Both of these issues are avoided by simply checking that the lane index
is a constant.

Fixes bug 35822.

Reviewers: t.p.northover, javed.absar

Subscribers: rengolin, kristof.beyls, mcrosier, llvm-commits

Differential Revision: https://reviews.llvm.org/D46591

llvm-svn: 332103