Add x86 tune to enable v2df vector reduction by paddpd.

The tune is disabled by default.

gcc/ChangeLog:

	PR target/97147
	* config/i386/i386.h (TARGET_V2DF_REDUCTION_PREFER_HADDPD):
	New macro.
	* config/i386/sse.md (*sse3_haddv2df3_low): Add
	TARGET_V2DF_REDUCTION_PREFER_HADDPD.
	(*sse3_hsubv2df3_low): Ditto.
	* config/i386/x86-tune.def
	(X86_TUNE_V2DF_REDUCTION_PREFER_HADDPD): New tune.

gcc/testsuite/ChangeLog:

	PR target/97147
	* gcc.target/i386/pr54400.c: Adjust testcase.
	* gcc.target/i386/pr94147.c: New test.