[RISCV] Narrow types of index operand matched pattern (shl (zext), C).

(shl (zext to iXLenVec), C) is a possible pattern in auto-vectorized code for
indexed loads/stores. But extending to iXLen might be too aggressive, RVV
indexed load/store instructions zero extend their indexed operand to XLEN.
The patch tries to narrow the type of the zero extension. It's benefit to
decrease register pressure.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D154687