[AArch64] Remove constraint strings from define_expand constructs

[AArch64] Remove constraint strings from define_expand constructs

A number of AArch64 define_expand patterns have specified constraints
for their operands. But the constraint strings are ignored at expand
time and are therefore redundant/useless. We now avoid specifying
constraints in new define_expands, but we should clean up the existing
define_expand definitions.

For example, the constraint "=w" is removed in the following case:
(define_expand "sqrt<mode>2"
    [(set (match_operand:GPF_F16 0 "register_operand" "=w")
The "" marks with an empty constraint in define_expand are removed as well.

2019-07-03  Dennis Zhang  <dennis.zhang@arm.com>

gcc/
	* config/aarch64/aarch64.md: Remove redundant constraints from
	define_expand but keep some patterns untouched if they are
	specially selected by TARGET_SECONDARY_RELOAD hook.
	* config/aarch64/aarch64-sve.md: Likewise.
	* config/aarch64/atomics.md: Remove redundant constraints from
	define_expand.
	* config/aarch64/aarch64-simd.md: Likewise.

From-SVN: r273021