i386: Add combine splitters to allow combining multiple insns into reg1 = const;...

i386: Add combine splitters to allow combining multiple insns into reg1 = const; reg2 = rotate (reg1, reg3 & cst) [PR96226]

As mentioned in the PR, we can combine ~(1 << x) into -2 r<< x, but we give
up in the ~(1 << (x & 31)) cases, as *<rotate_insn><mode>3_mask* don't allow
immediate operand 1 and find_split_point prefers to split (x & 31) instead
of the constant.

With these combine splitters we help combine decide how to split those
insns.

2020-12-04  Jakub Jelinek  <jakub@redhat.com>

	PR target/96226
	* config/i386/i386.md (splitter after *<rotate_insn><mode>3_mask,
	splitter after *<rotate_insn><mode>3_mask_1): New combine splitters.

	* gcc.target/i386/pr96226.c: New test.