store-merging: ICE in find_bswap_or_nop_1 PR95854.

store-merging: ICE in find_bswap_or_nop_1 PR95854.

The patch add a judgement in find_bswap_or_nop_1 to make sure both
operand1 and operand2 cannot be converted to unsigned HOST_WIDE_INT.
If not, return NULL.

gcc/ChangeLog:

2020-06-24  Kaipeng Zhou  <zhoukaipeng3@huawei.com>

	PR tree-optimization/95854
	* gimple-ssa-store-merging.c (find_bswap_or_nop_1): Return NULL
	if operand 1 or 2 of a BIT_FIELD_REF cannot be converted to
	unsigned HOST_WIDE_INT.

gcc/testsuite/ChangeLog:

2020-06-24  Kaipeng Zhou  <zhoukaipeng3@huawei.com>

	PR tree-optimization/95854
	* gcc.dg/pr95854.c: New test.