Adds LLVM version check to cmake.

We require LLVM 3.4 minimum, so if the user has something older make sure to not attempt compiling.
Fixes issues #7858 and #7906.
Merge pull request #1667 from Sonicadvance1/check_llvm_version

Adds LLVM version check to cmake.
Jit64: clean up casts in memory operands
Merge pull request #1668 from Tilka/memop_cleanup

Jit64: clean up casts in memory operands