[mlir] Add exp2 conversion to llvm.intr.exp2

[asmith]

Adding a test

[ftynse]

mlir/lib/Conversion/StandardToLLVM/StandardToLLVM.cpp

1227

Nit: this pattern was changed to be using Exp2OpLowering = VectorConvertToLLVMPattern<Exp2Op, LLVM::Exp2Op>;