[SandboxIR][NFC] Change order of ifs in Context::getOrCreateValueInternal()

Move the most common if statement to the top and the least common
ones to the bottom. This should save CPU cycles during compilation.

This patch also prefixes the llvm variables with the LLVM prefix
to make the naming convention in this function more uniform.
For example `C` to `LLVMC`.

Author: Vasileios Porpodas