[AArch64] Improve codegen of store lane instructions by avoiding GPR usage.

[AArch64] Improve codegen of store lane instructions by avoiding GPR usage.

We used to generate code similar to:

  umov.b        w8, v0[2]
  strb  w8, [x0, x1]

because the STR*ro* patterns were preferred to ST1*.
Instead, we can avoid going through GPRs, and generate:

  add   x8, x0, x1
  st1.b { v0 }[2], [x8]

This patch increases the ST1* AddedComplexity to achieve that.

rdar://16372710
Differential Revision: http://reviews.llvm.org/D6202

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@225183 91177308-0d34-0410-b5e6-96231b3b80d8