Remove unnecessary temporary in tree-if-conv.c

Remove unnecessary temporary in tree-if-conv.c

The call to ifc_temp_var in predicate_mem_writes become redundant
in r230099.  Before that point the mask was calculated using
fold_build_*s, but now it's calculated by gimple_build and so
is already a valid gimple value.

As it stands, the call forces an SSA_NAME-to-SSA_NAME copy
to be created, whereas SLP expects that such redundant copies
have already been eliminated.

2018-01-13  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* tree-if-conv.c (predicate_mem_writes): Remove redundant
	call to ifc_temp_var.

From-SVN: r256638