[AArch64][SVE] Always use overloaded methods instead of preprocessor macro.

This fixes a subtle issue where:

  svprf(pg, ptr, SV_ALL /*is sv_pattern instead of sv_prfop*/)

would be quietly accepted. With this change, the function declaration
guards that the third parameter is a `enum sv_prfop`. Previously `svprf`
would map directly to `__builtin_sve_svprfb`, which accepts the enum
operand as a signed integer and only checks that the incoming range is
valid, meaning that SV_ALL would be discarded as being outside the valid
immediate range, but would have allowed SV_VL1 without issuing a warning
(C) or error (C++).

Reviewed By: c-rhodes

Differential Revision: https://reviews.llvm.org/D100297