When merging many blocks to a single block (in the wrong order), instructions are getting moved over and over again.
This is quadratic and can result in very long compile times for large functions.
To fix this, always move the instruction to smaller block to the larger block.
rdar://problem/56268570
LLVM r356789 changed the format of textual IR to print nameless
blocks with labels instead of comments with "; <label>". Adjust Swift
tests to match. I also updated the utils/viewcfg script to match.
It is unfortunate that `Unsafe[Raw]BufferPointer._pointer` and
`baseAddress` are declared Optional. This leaves extra runtime checks
in the code in the most performance critical paths. Contrast this with
Array, which uses an sentinal pointer for the empty representation.
This forces us to use _unsafelyUnwrappedUnchecked whenever we just
want to dereference a non-empty buffer pointer.
Fixes SR-9809: swiftc appears to make some sub-optimal optimization choices
This saves a few instructions for some operations, like getting the count.
Also, it avoids the check for unwrapping the optional end pointer. For example, iterating over an unsafe buffer now has no overhead.
Also remove the _unboundedStartingAt initializer, which is not needed anymore.
