RISC-V: Clean up the pr106602.c testcase

RISC-V: Clean up the pr106602.c testcase

The test case that was added is rv64i-specific, as there's better ways
to generate this code on rv32i (where the long/int cast is a NOP) and on
rv64i_zba (where we have word shifts).  This renames the original test
case and adds two more for those targets.

gcc/testsuite/ChangeLog:
	PR target/106602
	* gcc.target/riscv/pr106602.c: Moved to...
	* gcc.target/riscv/pr106602-rv64i.c: ...here.
	* gcc.target/riscv/pr106602-rv32i.c: New test.
	* gcc.target/riscv/pr106602-rv64i_zba.c: New test.