[PATCH AArch64]Fix spurious warning with explicit initialization

[Bin Cheng]

Hi,
GCC gives spurious -Wmaybe-uninitialized message which breaks bootstrap.  This 
patch fixes it by explicitly initializing.  Also I file PR72355 in order to 
tracking the wrong warning message.
Compilation log checked.  Applied as obvious.

Thanks,
bin

2016-08-11  Bin Cheng  <bin.ch...@arm.com>

        * config/aarch64/aarch64-simd.md (vec_cmp<mode><v_cmp_result>: Init
        variable explicitly, also assert on it before use.
diff --git a/gcc/config/aarch64/aarch64-simd.md 
b/gcc/config/aarch64/aarch64-simd.md
index 37d397c..3817895 100644
--- a/gcc/config/aarch64/aarch64-simd.md
+++ b/gcc/config/aarch64/aarch64-simd.md
@@ -2413,7 +2413,7 @@
   enum rtx_code code = GET_CODE (operands[1]);
   rtx tmp = gen_reg_rtx (<V_cmp_result>mode);
 
-  rtx (*comparison) (rtx, rtx, rtx);
+  rtx (*comparison) (rtx, rtx, rtx) = NULL;
 
   switch (code)
     {
@@ -2495,6 +2495,7 @@
         a UNLE b -> !(a GT b)
         a UNLT b -> !(a GE b)
         a   NE b -> !(a EQ b)  */
+      gcc_assert (comparison != NULL);
       emit_insn (comparison (operands[0], operands[2], operands[3]));
       emit_insn (gen_one_cmpl<v_cmp_result>2 (operands[0], operands[0]));
       break;
@@ -2511,6 +2512,7 @@
         a LE b -> b GE a
         a LT b -> b GT a
         a EQ b -> a EQ b  */
+      gcc_assert (comparison != NULL);
       emit_insn (comparison (operands[0], operands[2], operands[3]));
       break;