testsuite: Don't test pr118149.c on AArch64

Recently two test cases for PR118149 have been added.
While pr118149-2.c works well for AArch64, pr118149.c fails
because the expected optimization in forwprop4 cannot be applied
as SLP vectorization does not happen.
This patch fixes this issue by disabling the check on AArch64.

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/pr118149.c: Disable for AArch64.

Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>