WIP: JitIL Float load/store optimizations. 

Ignore the first commit, it's #1075 which this depends on.

Improve the address generation code for floating point loads/stores to match integer loads/stores.
For floating loads with a known address, this eliminates the pattern of:

      mov r12d, 80001014
      mov rdx, r12d
      mov rdx, dword ptr [rbp+rdx]

and generates a nice simple:

      mov rdx, dword ptr [rbp+80001014]

Also fixed a bug in the PPC_FP code. I have no idea how broken it was, it was probably randoming picking the slow or fast path randomly depending on the value of RAX.