[SH] PR 51244 - Add -mzdcbranch option

[Oleg Endo]

Hello,

This adds a new SH option -mzdcbranch / -mnozdcbranch to tell the
compiler whether zero displacement conditional branches are fast on the
target CPU.  It is enabled by default for SH4* and thus does not change
any of the established behavior.
Currently this does affect only one pattern (negsi_cond).  However, I
intent to try using it for controlling conditional execution patterns in
the future (e.g. for PR 52628).

Tested with 'make all-gcc' and 'make info dvi pdf'.

Would that be OK?

Cheers,
Oleg

ChangeLog:

	PR target/51244
	* config/sh/sh.opt (mzdcbranch): New option.
	* doc/invoke.texi: Document it.
	* config/sh/sh.md (negsi_cond): Use TARGET_ZDCBRANCH as 
	condition instead of TARGET_HARD_SH4.
	* config/sh/sh.c (sh_option_override): Set TARGET_ZDCBRANCH as 
	default for TARGET_HARD_SH4.
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