[Clang][RISCV][RVV Intrinsic] Fix codegen redundant intrinsic names (#77889)

This patch avoids adding redundant vcreate_v intrinsics to the RISCV
IntrinsicList.
Since vcreate_v uses LFixedLog2LMUL, I believe we can simply set
Log2LMUL to the smallest value (-3) to prevent the creation of redundant
vcreate_v instances with the same intrinsic name and prototype in the
IntrinsicList when clang creates it.