[coroutines] Insert spills of PHI instructions correctly

Summary:
Fix a bug where we were inserting a spill in between the PHIs in the beginning of the block.
Consider this fragment:

```
begin:
  %phi1 = phi i32 [ 0, %entry ], [ 2, %alt ]
  %phi2 = phi i32 [ 1, %entry ], [ 3, %alt ]
  %sp1 = call i8 @llvm.coro.suspend(token none, i1 false)
  switch i8 %sp1, label %suspend [i8 0, label %resume
                                  i8 1, label %cleanup]
resume:
  call i32 @print(i32 %phi1)
```
Unless we are spilling the argument or result of the invoke, we were always inserting the spill immediately following the instruction.
The fix adds a check that if the spilled instruction is a PHI Node, select an appropriate insert point with `getFirstInsertionPt()` that
skips all the PHI Nodes and EH pads.

Reviewers: majnemer, rnk

Reviewed By: rnk

Subscribers: qcolombet, EricWF, llvm-commits

Differential Revision: https://reviews.llvm.org/D31799

llvm-svn: 299771