Improve aarch64_legitimate_constant_p

Improve aarch64_legitimate_constant_p

This patch further improves aarch64_legitimate_constant_p.  Allow all
integer, floating point and vector constants.  Allow label references
and non-anchor symbols with an immediate offset.  This allows such
constants to be rematerialized, resulting in smaller code and fewer stack
spills.  SPEC2006 codesize reduces by 0.08%, SPEC2017 by 0.13%.

    gcc/
	* config/aarch64/aarch64.c (aarch64_legitimate_constant_p):
	Return true for more constants, symbols and label references.
	(aarch64_valid_floating_const): Remove unused function.

From-SVN: r254388