Allow SCmode and DImode to be tieable with TARGET_64BIT on x86_64.

This patch is a form of insurance policy in case my patch for PR 7061 runs
into problems on non-x86 targets; the middle-end can add an extra check
that the backend is happy placing SCmode and DImode values in the same
register, before creating a SUBREG.  Unfortunately, ix86_modes_tieable_p
currently claims this is not allowed(?), even though the default target
hook for modes_tieable_p is to always return true [i.e. false can be
used to specifically prohibit bad combinations], and the x86_64 ABI
passes SCmode values in DImode registers!.  This makes the backend's
modes_tiable_p hook a little more forgiving, and additionally enables
interconversion between SCmode and V2SFmode, and between DCmode and
VD2Fmode, which opens interesting opporutunities in the future.

2022-05-30  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* config/i386/i386.cc (ix86_modes_tieable_p): Allow SCmode to be
	tieable with DImode on TARGET_64BIT, and SCmode tieable with
	V2SFmode, and DCmode with V2DFmode.