gcc: xtensa: fix PR target/98285

2020-12-14  Max Filippov  <jcmvbkbc@gmail.com>
gcc/
	* config/xtensa/predicates.md (addsubx_operand): Change accepted
	values from 2/4/8 to 1..3.
	* config/xtensa/xtensa.md (*addx, *subx): Change RTL pattern
	to use 'ashift' instead of 'mult'. Update operands[3] value.

gcc/testsuite/
	* gcc.target/xtensa/pr98285.c: New test.