PR target/98743: Fix ICE in convert_move for RISC-V

PR target/98743: Fix ICE in convert_move for RISC-V

 - Check `from` mode is not BLMmode before call store_expr, calling store_expr
   with BLKmode will cause ICE.

 - Verified with riscv64, x86_64 and aarch64, no introduce new regression.

Note: Those logic was introduced by 3e60ddeb8220ed388819bb3f14e8caa9309fd3c2,
      so I cc Jakub for reivew.

Changes for V2:

 - Checking mode of `from` rather than mode of `to`.
 - Verified on riscv64, x86_64 and aarch64 again.

gcc/ChangeLog:

	PR target/98743
	* expr.c: Check mode before calling store_expr.

gcc/testsuite/ChangeLog:

	PR target/98743
	* g++.dg/opt/pr98743.C: New.