i386: Use plus_constant instead of gen_rtx_PLUS

i386: Use plus_constant instead of gen_rtx_PLUS

Replace gen_rtx_PLUS with a GEN_INT with plus_constant.

2020-05-03  Uroš Bizjak  <ubizjak@gmail.com>

        * config/i386/i386-expand.c (ix86_expand_int_movcc):
	Use plus_constant instead of gen_rtx_PLUS with GEN_INT.
	(emit_memmov): Ditto.
	(emit_memset): Ditto.
	(ix86_expand_strlensi_unroll_1): Ditto.
	(release_scratch_register_on_entry): Ditto.
	(gen_frame_set): Ditto.
	(ix86_emit_restore_reg_using_pop): Ditto.
	(ix86_emit_outlined_ms2sysv_restore): Ditto.
	(ix86_expand_epilogue): Ditto.
	(ix86_expand_split_stack_prologue): Ditto.
	* config/i386/i386.md (push immediate splitter): Ditto.
	(strmov): Ditto.
	(strset): Ditto.