[Clang][RISCV] Update operand order for vmerge and vcompress

From:
  vint32m1_t vmerge_vvm_i32m1 (vbool32_t mask, vint32m1_t op1, vint32m1_t op2, size_t vl);
  vint32m1_t vcompress_vm_i32m1 (vbool32_t mask, vint32m1_t src, size_t vl);

To:
  vint32m1_t vmerge_vvm_i32m1 (vint32m1_t op1, vint32m1_t op2, vbool32_t selector, size_t vl);
  vint32m1_t vcompress_vm_i32m1 (vint32m1_t src, vbool32_t selector, size_t vl);

Address issues:
riscv-non-isa/rvv-intrinsic-doc#140
riscv-non-isa/rvv-intrinsic-doc#167

Pull request:
riscv-non-isa/rvv-intrinsic-doc#185

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D140686