[GCC 10][committed] aarch64: Add internal tune flag to minimise VL-based scalar ops

[Kyrylo Tkachov]

This is a backport of the cse_sve_vl_constants tuning param to GCC 10.
It's had a couple of trivial merge conflicts to resolve.

Bootstrapped and tested on the branch on aarch64-none-linux-gnu.
Pushing to the branch.

Thanks,
Kyrill

gcc/ChangeLog:

	* config/aarch64/aarch64-tuning-flags.def (cse_sve_vl_constants):
	Define.
	* config/aarch64/aarch64.md (add<mode>3): Force CONST_POLY_INT immediates
	into a register when the above is enabled.
	* config/aarch64/aarch64.c (neoversev1_tunings):
	AARCH64_EXTRA_TUNE_CSE_SVE_VL_CONSTANTS.
	(aarch64_rtx_costs): Use AARCH64_EXTRA_TUNE_CSE_SVE_VL_CONSTANTS.

gcc/testsuite/

	* gcc.target/aarch64/sve/cse_sve_vl_constants_1.c: New test.
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