Limit the number of iterations in phi analyzer.

The phi analyzer attempts to provide initial values for cyclic PHIs by
simulating the modifer by the number of bits of precision.  _BitInt
objects can be very large, and it is pointless to try to simulate them
that many ttimes.

	PR tree-optimization/126538
	gcc/
	* gimple-range-phi.cc (phi_group::calculate_using_modifier): Limit the
	iteration upper bound to MAX_FIXED_MODE_SIZE.

	gcc/testsuite/
	* gcc.dg/pr126538.c: New.