gcc: Fix internal compiler error for PPC test case "gcc.dg/vmx/7d-02.c"

gcc: Fix internal compiler error for PPC test case "gcc.dg/vmx/7d-02.c"

Fix an internal compiler error on PPC from building a specific test:

$ $CC -S 7d-02.c
7d-02.c: In function â:
7d-02.c:11:5: internal compiler error: in copy_to_mode_reg, at explow.c:612
     vec_st(v, i*16,p);
     ^~~~~~

The failure appears to happen on all optimizations levels as well.

Signed-off-by: Mark Hatle <mark.hatle@windriver.com>
Signed-off-by: Armin Kuster <akuster808@gmail.com>