[FuzzMutate] Skip EHPad during mutation and avoid replacing callee with pointer when...

[FuzzMutate] Skip EHPad during mutation and avoid replacing callee with pointer when sinking

This patch addresses 2 problems:

- In `ShuffleBlockStrategy`, when `BB` is an EHPad, `BB.getFirstInsertionPt()` will return `BB.end()`, which cannot be dereferenced and will cause crash in following loop.
- In `isCompatibleReplacement`, a call instruction's callee might be replaced by a pointer, causing 2 subproblems:
  - we cannot guarantee that the pointer is a function pointer (even if it is, we cannot guarantee it matches the signature).
  - after such a replacement, `getCalledFunction` will from then on return `nullptr` (since it's indirect call) which causes Segmentation Fault in the lines below.

This patch fixes the first problem by checking if a block to be mutated is an EHPad in base class `IRMutationStrategy` and skipping mutating it if so.

This patch fixes the second problem by avoiding replacing callee with pointer and adding a null check for indirect calls.

Reviewed By: Peter

Differential Revision: https://reviews.llvm.org/D148853