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.