lower-bitint: Fix up handling of uninitialized large/huge _BitInt call arguments [PR113316]
The code to assign large/huge _BitInt SSA_NAMEs to partitions intentionally
ignores uninitialized SSA_NAMEs:
/* Also ignore uninitialized uses. */
if (SSA_NAME_IS_DEFAULT_DEF (s)
&& (!SSA_NAME_VAR (s) || VAR_P (SSA_NAME_VAR (s))))
continue;
because there is no need to store them into memory, all we need is when
trying to extract some limb from them use uninitialized SSA_NAME for the
limb.
The following testcase shows this is a problem for call arguments though,
for those we need to create replacement SSA_NAMEs which are loaded from
the underlying variable. For uninitialized SSA_NAMEs because we didn't
create underlying variable for them var_to_partition doesn't work, the
following patch handles it by just creating an uninitialized replacement
SSA_NAME.