[4.2] SILCombine: remove a problematic cast peephole optimization.

[eeckstein]

This optimization inserted bit casts based on structural properties of types.
It caused a miscompile in case of imported C structs. Also, by inspection, I found that checks for resilient types are missing.
As this optimization does not have any noticeable impact on the benchmarks it's better to remove it at all, together with the complexity for checking the types.

rdar://problem/40074362

[eeckstein]

@swift-ci test

[eeckstein]

Explanation: A specific peephole optimization in SILCombine did not deal with Clang imported structs. This caused a miscompile and a runtime crash. As this optimization involves complex code and does not give any noticeable performance improvements, this change completely removes the optimization.
Scope: It affects code which casts from/to Clang imported types, like with the UnsafePointer APIs.
Radar/SR Issue: rdar://problem/40074362
Risk: Low: The change just removes the optimization. We also verified that removing the optimization does not introduce a performance/code size regression.
Testing: Tested with lit tests. The performance/size impact was tested with the swift benchmark suite and with the source compatibility projects.
Reviewer: @atrick

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 4e4080ed124bda225f8bdf0193d9d6ba16a05fc1

[eeckstein]

@swift-ci test

[eeckstein]

@swift-ci test