The static Opcode and Emitter members of the InstImpl<> template class
have constant values, so we can make them constexpr and define them in
the header.

Note that we can't make them constexpr in the template class definition,
since that requires in-class initialization, and GCC considers the
definition of the static members of the concrete instantiations to be
doing duplicate initialization (this might be a compiler bug, since
Clang and MSVC allow it). Fortunately, constexpr is a specifier, which
implies const, a qualifier, meaning a definition which is constexpr is
compatible with a declaration which is only const. Declaring them as
const in the class has the added benefit that if we instantiated the
template without providing initialization of these members, we'd get a
compilation error instead of using the in-class initializer of the
primary template.

Note also that defining these static members in the header doesn't lead
to multiple definition errors because in C++17, static constexpr member
variables are implicitly inline.

Lastly, constexpr applied to a pointer affects the pointer value itself,
not the pointee, so "constexpr const char *s" is a constexpr pointer to
const characters. Thus while constexpr implies const, the second const
isn't redundant because it applies to the pointee type.

Bug: b/192890685
Change-Id: Ia7c6a889d08007d42393979f3b08bcfc5fa8c614
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/55428
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>