It only contained the now trivial OpcodeName() implementation. Since the
SpirvShaderInstructions.cpp file already contains other SPIR-V opcode
utility functions, move it there.

Also make this function return a pointer to a constant char string,
to avoid needless std::string construction.

Bug: b/145351270
Change-Id: Iacfc887566b0a9057f37391ad52d43cdca912308
Signed-off-by: Nicolas Capens <capn@google.com>
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/51448
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>