expr: Fix ICE on BFmode -> SFmode conversion of constant [PR107262]

expr: Fix ICE on BFmode -> SFmode conversion of constant [PR107262]

I forgot to handle the case where lowpart_subreg returns a VOIDmode
CONST_INT, in that case convert_mode_scalar obviously doesn't work.

The following patch fixes that.

2022-10-19  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/107262
	* expr.cc (convert_mode_scalar): For BFmode -> SFmode conversions
	of constants, use simplify_unary_operation if fromi has VOIDmode
	instead of recursive convert_mode_scalar.

	* gcc.dg/pr107262.c: New test.