[ASan] Minor fixes to dynamic allocas handling:

* Don't instrument promotable dynamic allocas:
  We already have a test that checks that promotable dynamic allocas are
  ignored, as well as static promotable allocas. Make sure this test will
  still pass if/when we enable dynamic alloca instrumentation by default.

* Handle lifetime intrinsics before handling dynamic allocas:
  lifetime intrinsics may refer to dynamic allocas, so we need to emit
  instrumentation before these dynamic allocas would be replaced.

Differential Revision: http://reviews.llvm.org/D12704

llvm-svn: 251045