GBE: Fix a bug in encoding MATH instruction

GBE: Fix a bug in encoding MATH instruction

For std::vector, a push_back may cause memory relocation if no enough
memory in the vector pool. And iterator or pointer got previously will
become invalid after relocation.

Here in GenEncoder::next(), which will call push_back(), memory
relocation may occur. Then relocation will make 'insn' point to
invalid memory that does not belong to GenEncoder::store anymore.
Then, the setting of execution_width will fail.

Signed-off-by: Ruiling Song <ruiling.song@intel.com>
Reviewed-by: Zhigang Gong <zhigang.gong@linux.intel.com>