[mono] Merge LLVM and non-LLVM code paths in emit_vector_2_3_4

[LeVladIonescu]

Currently, there are a few code paths in emit_vector_2_3_4 that are quite similar and could be merged in order to simplify the code.

i.e. Eliminate this type of if-else statement

if (COMPILE_LLVM (CFG) { // llvm code path } else { // non-llvm code path }