[FPEnv] Get rid of extra moves in fpenv calls

If intrinsic `get_fpenv` or `set_fpenv` is lowered to the form where FP
environment is represented as a region in memory, extra moves can
appear. For example the code:

  define void @func_01(ptr %ptr) {
    %env = call i256 @llvm.get.fpenv.i256()
    store i256 %env, ptr %ptr
    ret void
  }

produces DAG:

  ch = get_fpenv_mem ch, memory_region
  val: i256, ch = load ch, memory_region
  ch = store ch, ptr, val

In this case the extra moves can be avoided if `get_fpenv_mem` got
pointer to the memory where the FP environment should be finally placed.

This change implement such optimization for this use case.

Differential Revision: https://reviews.llvm.org/D150437

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