riscv: Fix up riscv_rtx_costs for RTL checking (PR target/93333)

riscv: Fix up riscv_rtx_costs for RTL checking (PR target/93333)

As mentioned in the PR, during combine rtx_costs can be called sometimes
even on RTL that has not been validated yet and so can contain even operands
that aren't valid in any instruction.

2020-01-21  Jakub Jelinek  <jakub@redhat.com>

	PR target/93333
	* config/riscv/riscv.c (riscv_rtx_costs) <case ZERO_EXTRACT>: Verify
	the last two operands are CONST_INT_P before using them as such.

	* gcc.c-torture/compile/pr93333.c: New test.