swift234

IRGen: Emit failure blocks individually

This significantly improves debugging experience as failures such as overflow
can be mapped back to a source line.

Code size impacts I measured on PerfTestSuite and libswift*.dylib was
neglectable.

__text section size increase:

  0.24% bin/PerfTest_O
  0.01% bin/PerfTest_Onone
  0.20% libswift*.dylib

rdar://19118593

Swift SVN r24722