[AArch64] Fix incorrect LD1 of 16-bit FP vectors in big endian

[AArch64] Fix incorrect LD1 of 16-bit FP vectors in big endian

Summary:
Loading a vector of 4 half-precision FP sometimes results in an LD1
of 2 single-precision FP + a reversal. This results in an incorrect
byte swap due to the conversion from little endian to big endian.

In order to generate the correct byte swap, it is easier to
generate the correct LD1 of 4 half-precision FP, thus avoiding the
subsequent reversal.

Reviewers: craig.topper, jmolloy, olista01

Reviewed By: olista01

Subscribers: efriedma, samparker, SjoerdMeijer, rogfer01, aemerson, rengolin, javed.absar, kristof.beyls, llvm-commits

Differential Revision: https://reviews.llvm.org/D41863

llvm-svn: 322663