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++).