match.pd: Optimize (((type)A * B) >> prec) != 0 into __imag__ .MUL_OVERFLOW [PR94914]

match.pd: Optimize (((type)A * B) >> prec) != 0 into __imag__ .MUL_OVERFLOW [PR94914]

On x86 (the only target with umulv4_optab) one can use mull; seto to check
for overflow instead of performing wider multiplication and performing
comparison on the high bits.

2020-05-05  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/94914
	* match.pd ((((type)A * B) >> prec) != 0 to .MUL_OVERFLOW(A, B) != 0):
	New simplification.

	* gcc.target/i386/pr94914.c: New test.