bitint: Fix up handling of uninitialized mul/div/float cast operands [PR121127]

bitint: Fix up handling of uninitialized mul/div/float cast operands [PR121127]

handle_operand_addr (used for the cases where we use libgcc APIs, so
multiplication, division, modulo, casts of _BitInt to float/dfp) when it
sees default definition of an SSA_NAME which is not PARM_DECL (i.e.
uninitialized one) just allocates single uninitialized limb, there is no
need to waste more memory on it, it can just tell libgcc that it has
64-bit precision, not say 1024 bit etc.
Unfortunately, doing this runs into some asserts when we have a narrowing
cast of the uninitialized SSA_NAME (but still large/huge _BitInt).

The following patch fixes that by using a magic value in *prec_stored
for the uninitialized cases (0) and just don't do any *prec tweaks for
narrowing casts from that.  precs still needs to be maintained as before,
that one is used for big endian adjustment.

2025-08-06  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/121127
	* gimple-lower-bitint.cc (bitint_large_huge::handle_operand_addr): For
	uninitialized SSA_NAME, set *prec_stored to 0 rather than *prec.
	Handle that case in narrowing casts.  If prec_stored is non-NULL,
	set *prec_stored to prec_stored_val.

	* gcc.dg/bitint-125.c: New test.