lower-bitint: Fix up .{ADD,SUB}_OVERFLOW lowering

torture/bitint-37.c test FAILed on i686-linux e.g. on
signed _BitInt(575) + unsigned _BitInt(575) -> signed _BitInt(575)
__builtin_add_overflow.  With 64-bit limbs, we use 4 .UADDC calls in
the IL, 2 in a loop (which handles the first 8 limbs), then one partial
limb (we use 63 bits from that) and finally last_ovf case due to the
mixing of signed vs. unsigned.
But with 32-bit limbs, we use 5 .UADDC calls in the IL, 2 in a loop (which
handles the first 16 limbs), then one full limb above that, one partial
(31 bits) and finally last_ovf case, and for the last_ovf case the code
computed incorrect idx and so partly did the wrong thing, e.g. overwrote
the result from the previous .UADDC.

Fixed thusly.

2024-02-10  Jakub Jelinek  <jakub@redhat.com>

	* gimple-lower-bitint.cc (itint_large_huge::lower_addsub_overflow): Fix
	computation of idx for i == 4 of bitint_prec_huge.