Fix HostReg enum cast outside valid range

[j8r]

LLVM 16 errors by reporting that the valid range of HostReg is between 0 and 15.

I don't have enough knowledge if this is the best fix. Using a different type accepting the 16 value may be another option. I changed to an unsigned type.

Here is the full error:

/duckstation/src/core/cpu_recompiler_types.h:76:37: error: integer value 16 is outside the valid range of values [0, 15] for this enumeration type [-Wenum-constexpr-conversion]
constexpr HostReg HostReg_Invalid = static_cast<HostReg>(HostReg_Count);
                                    ^

[stenzek]

I had already changed this as part of bf15d13 yesterday for clang-cl compatibility, just hadn't pushed it yet.