Correctly implement SILBasicBlock::erase().
Correctly implement SILBasicBlock::erase().

LLVM ilist::erase does not return the following iterator the way an STL list
would. Compensate by computing the next iterator and ignoring LLVM's result.

I expect this to fix the ASAN bots:
<rdar://problem/31550303> OSS Swift CI ASAN bot fails on AccessMarker tests.

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