Fix incorrect optimization by cprop_hardreg.

Fix incorrect optimization by cprop_hardreg.

If SRC had been assigned a mode narrower than the copy, we can't
always link DEST into the chain even they have same
hard_regno_nregs(i.e. HImode/SImode in i386 backend).

i.e
        kmovw   %k0, %edi
        vmovd   %edi, %xmm2
	vpshuflw        $0, %xmm2, %xmm0
        kmovw   %k0, %r8d
        kmovd   %k0, %r9d
...
-	 movl %r9d, %r11d
+	 vmovd %xmm2, %r11d

gcc/ChangeLog:

	PR rtl-optimization/98694
	* regcprop.c (copy_value): If SRC had been assigned a mode
	narrower than the copy, we can't link DEST into the chain even
	they have same hard_regno_nregs(i.e. HImode/SImode in i386
	backend).

gcc/testsuite/ChangeLog:

	PR rtl-optimization/98694
	* gcc.target/i386/pr98694.c: New test.