[AArch64]Fix a problem that the register order of fmls/fmla by element is incorrect.

E.g. the codegen result is 
     fmls v1.2s, v0.2s, v2.s[3]
which is expected to be
     fmls v0.2s, v1.2s, v2.s[3]

llvm-svn: 198001