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