[RISCV] Correct resource cycles for vzext/vsext in SiFive7 scheduler.

The instructions produce DLEN bits per cycle. The vsetvli LMUL for these
instructions is the output EMUL. The input EMUL is scaled down by
the vector factor suffix on the instruction name.

So for LMUL=1 there are 2*DLEN bits of result produced over 2 cycles.
This makes SiFive7GetCyclesDefault the correct resource cycles.

Reviewed By: monkchiang

Differential Revision: https://reviews.llvm.org/D155010