[AArch64] Optimize floating point materialization

This patch changes isFPImmLegal to return if the value can be enconded
as the immediate operand of a logical instruction besides checking if
for immediate field for fmov.

This optimizes some floating point materization, inclusive values
used on isinf lowering.

Reviewed By: rengolin, efriedma, evandro

Differential Revision: https://reviews.llvm.org/D57044

llvm-svn: 352866