[RISCV] Support vector type strict_fma.

Like D145900, the patch also supports fixed vector strict_fma nodes in RISC-V by
customized lowering them to riscv_strict_vfmadd_vl nodes. riscv_strict_vfmadd_vl
is created to avoid some riscv_vfmadd_vl optimizations happening to original
strict_fma nodes. The patch also adds combine patterns for riscv_strict_fmadd_vl
nodes with negation operands.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D146939