[RISCV] Fix the bug in the register allocator caused by reserved BP.

[RISCV] Fix the bug in the register allocator caused by reserved BP.

Originally, hasRVVFrameObject() will scan all the stack objects to check
whether if there is any scalable vector object on the stack or not.
However, it causes errors in the register allocator. In issue 53016, it
returns false before RA because there is no RVV stack objects. After RA,
it returns true because there are spilling slots for RVV values during RA.
The compiler will not reserve BP during register allocation and generate BP
access in the PEI pass due to the inconsistent behavior of the function.

The function is changed to use hasStdExtV() as the return value. It is
not precise, but it can make the register allocation correct.

Refer to https://github.com/llvm/llvm-project/issues/53016.

Differential Revision: https://reviews.llvm.org/D117663