lower-bitint: Fix handle_cast when used e.g. in comparisons of precisions...

lower-bitint: Fix handle_cast when used e.g. in comparisons of precisions multiple of limb_prec [PR113849]

handle_cast handles the simple way all narrowing large/huge bitint to
large/huge bitint conversions and also such widening conversions if we can
assume that the most significant limb is processed using constant index
and both lhs and rhs have same number of limbs.
But, the condition whether we can rely on the most significant limb
being processed using constant index is incorrect.
For m_upwards_2limb it was correct (m_upwards_2limb then is the number
of limbs handled by the loop, so if lhs_type has larger precision than
that, it is handled with constant index), similarly if m_var_msb is set
(on left shifts), it is never handled with constant idx.  But in other
cases, like right shifts or non-equality comparisons, or bitquery operations
which operate from most significant to least significant limb, all those
can handle even the most significant limb in a loop when lhs_type has
precision which is a multiple of limb_prec.

So, the following patch punts on the optimization in that case and goes for
the conditionals in the loop for that case.

2024-02-12  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/113849
	* gimple-lower-bitint.cc (bitint_large_huge::handle_cast): Don't use
	fast path for widening casts where !m_upwards_2limb and lhs_type
	has precision which is a multiple of limb_prec.

	* gcc.dg/torture/bitint-58.c: New test.