[RISCV] RISCV vector calling convention (1/2) (#77560)

    [RISCV] RISCV vector calling convention (1/2)

    This is the vector calling convention based on
    https://github.com/riscv-non-isa/riscv-elf-psabi-doc,
    the idea is to split between "scalar" callee-saved registers
    and "vector" callee-saved registers. "scalar" ones remain the
    original strategy, however, "vector" ones are handled together
    with RVV objects.

    The stack layout would be:

      |--------------------------| <-- FP
      | callee-allocated save    |
      | area for register varargs|
      |--------------------------|
      | callee-saved registers   | <-- scalar callee-saved
      |        (scalar)          |
      |--------------------------|
      | RVV alignment padding    |
      |--------------------------|
      | callee-saved registers   | <-- vector callee-saved
      |        (vector)          |
      |--------------------------|
      | RVV objects              |
      |--------------------------|
      | padding before RVV       |
      |--------------------------|
      | scalar local variables   |
      |--------------------------| <-- BP
      | variable size objects    |
      |--------------------------| <-- SP

    Note: This patch doesn't contain "tuple" type, e.g. vint32m1x2.
          It will be handled in https://github.com/riscv-non-isa/riscv-elf-psabi-doc (2/2).

    Differential Revision: https://reviews.llvm.org/D154576

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