[RISCV][NFCI] Set TransientStackAlignment and rely on it rather than...

[RISCV][NFCI] Set TransientStackAlignment and rely on it rather than RVV-specific logic on RVV-less functions

* TargetFrameLowering has a TransientStackAlignment field that "returns
  the number of bytes to which the stack pointer must be aligned at all
  times, even between calls.
  * As explained in the [RISC-V calling
    convention](https://github.com/riscv-non-isa/riscv-elf-psabi-doc/blob/master/riscv-cc.adoc),
    the stack pointer must remain fully aligned throughout execution for
    compliant code. This is important for embedded targets that might avoid
    realigning the stack pointer for interrupt service routines. Systems
    running full OSes may always realign the stack anyway.
* TransientStackAlignment is used in estimateStackSize in
  MachineFrameInfo and in PEI::calculateFrameObjectOffsets.
  * estimateStackSize is only used in the RISC-V backend for scavenging
    slots. It may be possible to craft a function where the difference
    is observable, but it wouldn't be a meaningful test.
  * calculateFrameObjectOffsets makes use of TransientStackAlignment,
    but then sets the stack alignment to the max of that alignment and
    MaxAlign, which is unconditionally set to 16 in
    RISCVFrameLowering::processFunctionBeforeFrameFinalized
  * I've changed this logic to only set MaxAlign if there are RVV frame
    objects. There should be no functional change here for either RVV
    targets (MaxAlign is set as before) or non-RVV targets
    (TransientStackAlign is now 16 anyway).

Differential Revision: https://reviews.llvm.org/D130068