[RISCV] Remove -riscv-insert-vsetvl-strict-asserts flag (#90171)

This flag has been enabled by default for almost two years now since
1f06398e, and at this stage we probably
shouldn't be falling back to the fixups.

This removes the flag so we always perform the assertion, as well as
making sure that CurInfo is always valid on exit: We shouldn't leave
emitVSETVLIs with an uninitialized VSETVLIInfo.