[RISCV] Disable lax vector conversions between RVVBuiltin types and RVVFixedLengthDataVector.

This seems to be causing issues with using overloaded RVV intrinsics
that take scalar operands. If the scalar type passed in doesn't exactly
match the element type.

I blindly copied this feature from SVE. Since no one has asked for it
I'd prefer to remove it to make overloaded intrinsics work as expected.

By removing the lax conversions, types declared with __attribute__((riscv_rvv_vector_bits(__riscv_v_fixed_vlen)))
can only ever be used like their underlying RVV builtin type. No lax conversions
to other element sizes with the same LMUL.

Fixes #64404.