rtl: Validate subreg info when optimizing vec_select.

rtl: Validate subreg info when optimizing vec_select.

When optimizing for NOPs in case of overlapping regs in VEC_SELECT expressions,
validate subreg data before using simplify_subreg_regno.  There is no real
SUBREG rtx here, but a pseudo subreg call to check if subregs are possible.

gcc/ChangeLog:

	* rtlanal.cc (set_noop_p): Validate subreg constraints before checking
	for overlapping regs using simplify_subreg_regno.