[AArch64] Split 0 vector stores into scalar store pairs.

Summary:
Replace a splat of zeros to a vector store by scalar stores of WZR/XZR.
The load store optimizer pass will merge them to store pair stores.
This should be better than a movi to create the vector zero followed by
a vector store if the zero constant is not re-used, since one
instructions and one register live range will be removed.

For example, the final generated code should be:

  stp xzr, xzr, [x0]

instead of:

  movi v0.2d, #0
  str q0, [x0]

Reviewers: t.p.northover, mcrosier, MatzeB, jmolloy

Subscribers: aemerson, rengolin, llvm-commits

Differential Revision: https://reviews.llvm.org/D26561

llvm-svn: 286875