[AArch64] Remove unneeded FSUB alternatives and add a new one

[AArch64] Remove unneeded FSUB alternatives and add a new one

The floating-point subtraction patterns don't need to handle
subtraction of constants, since those go through the addition
patterns instead.  There was a missing MOVPRFX alternative for
FSUBR though.

2019-08-15  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* config/aarch64/aarch64-sve.md (*sub<SVE_F:mode>3): Remove immediate
	FADD and FSUB alternatives.  Add a MOVPRFX alternative for FSUBR.

From-SVN: r274514