[AArch64] Add support for 16-bit FMOV immediates

[AArch64] Add support for 16-bit FMOV immediates

aarch64_float_const_representable_p was still returning false for
HFmode, so we wouldn't use 16-bit FMOV immediate.  E.g. before the
patch:

    __fp16 foo (void) { return 0x1.1p-3; }

gave:

       mov     w0, 12352
       fmov    h0, w0

with -march=armv8.2-a+fp16, whereas now it gives:

       fmov    h0, 1.328125e-1

2018-08-02  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* config/aarch64/aarch64.c (aarch64_float_const_representable_p):
	Allow HFmode constants if TARGET_FP_F16INST.

gcc/testsuite/
	* gcc.target/aarch64/f16_mov_immediate_1.c: Expect fmov immediate
	to be used.
	* gcc.target/aarch64/f16_mov_immediate_2.c: Likewise.
	* gcc.target/aarch64/f16_mov_immediate_3.c: Force +nofp16.
	* gcc.target/aarch64/sve/single_1.c: Except fmov immediate to be used
	for .h.
	* gcc.target/aarch64/sve/single_2.c: Likewise.
	* gcc.target/aarch64/sve/single_3.c: Likewise.
	* gcc.target/aarch64/sve/single_4.c: Likewise.

From-SVN: r263250