builtins: Fix up expand_builtin_int_roundingfn_2 [PR105211]

builtins: Fix up expand_builtin_int_roundingfn_2 [PR105211]

The expansion of __builtin_iround{,f,l} etc. builtins in some cases
emits calls to a different fallback builtin.  To locate the right builtin
it uses mathfn_built_in_1 with the type of the first argument.
If its TYPE_MAIN_VARIANT is {float,double,long_double}_type_node, all is
fine, but on the following testcase, because GIMPLE considers scalar
float conversions between types with the same mode as useless,
TYPE_MAIN_VARIANT of the arg's type is float32_type_node and because there
isn't __builtin_lroundf32 returns NULL and we ICE.

This patch will first try the type of the first argument of the builtin's
prototype (so that say on sizeof(double)==sizeof(long double) target it honors
whether it was a *l or non-*l call; though even that can't be 100% trusted,
user could incorrectly prototype it) and as fallback the type argument.
If neither works, doesn't fallback.

2022-04-11  Jakub Jelinek  <jakub@redhat.com>

	PR rtl-optimization/105211
	* builtins.cc (expand_builtin_int_roundingfn_2): If mathfn_built_in_1
	fails for TREE_TYPE (arg), retry it with
	TREE_VALUE (TYPE_ARG_TYPES (TREE_TYPE (fndecl))) and if even that
	fails, emit call normally.

	* gcc.dg/pr105211.c: New test.