vregs: Use force_subreg when instantiating subregs [PR120721]

vregs: Use force_subreg when instantiating subregs [PR120721]

In this PR, we started with:

    (subreg:V2DI (reg:DI virtual-reg) 0)

and vregs instantiated the virtual register to the argument pointer.
But:

    (subreg:V2DI (reg:DI ap) 0)

is not a sensible subreg, since the argument pointer certainly can't
be referenced in V2DImode.  This is (IMO correctly) rejected after
g:2dcc6dbd8a00caf7cfa8cac17b3fd1c33d658016.

The vregs code that instantiates the subreg above is specific to
rvalues and already creates new instructions for nonzero offsets.
It is therefore safe to use force_subreg instead of simplify_gen_subreg.

I did wonder whether we should instead say that a subreg of a
virtual register is invalid if the same subreg would be invalid
for the associated hard registers.  But the point of virtual registers
is that the offsets from the hard registers are not known until after
expand has finished, and if an offset is nonzero, the virtual register
will be instantiated into a pseudo that contains the sum of the hard
register and the offset.  The subreg would then be correct for that
pseudo.  The subreg is only invalid in this case because there is
no offset.

gcc/
	PR rtl-optimization/120721
	* function.cc (instantiate_virtual_regs_in_insn): Use force_subreg
	instead of simplify_gen_subreg when instantiating an rvalue SUBREG.

gcc/testsuite/
	PR rtl-optimization/120721
	* g++.dg/torture/pr120721.C: New test.