[AArch64] Avoid generating indexed vector instructions for Exynos

Avoid generating indexed vector instructions for Exynos. This is needed for
fmla/fmls/fmul/fmulx. For example, the instruction

  fmla v0.4s, v1.4s, v2.s[1]

is less efficient than the instructions

  dup v2.4s, v2.s[1]
  fmla v0.4s, v1.4s, v2.4s

Patch written by Abderrazek Zaafrani.

Differential Revision: https://reviews.llvm.org/D21571

llvm-svn: 283663