[mlir][LLVMIR] Add support for translating FCmp & FP constants

This patch add supports for translating FCmp and more kinds of FP
constants in addition to 32 & 64-bit ones. However, we can't express
ppc_fp128 constants right now because the semantics for its underlying
APFloat is `S_PPCDoubleDouble` but mlir::FloatType doesn't support such
semantics right now.

Differential Revision: https://reviews.llvm.org/D124630