Re-commit r220667.

C++ source given to check_cxx_source_compile should have define "main".
[asan-asm-instrumentation] Added comment describing how asm instrumentation works.

Summary: [asan-asm-instrumentation] Added comment describing how asm instrumentation works.

Reviewers: eugenis

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D5970
[ARM] Select VMAXNM and VMINNM regardless of operand order

Currently, the ARM backend will select the VMAXNM and VMINNM for these C
expressions:
  (a < b) ? a : b
  (a > b) ? a : b
but not these expressions:
  (a > b) ? b : a
  (a < b) ? b : a

This patch allows all of these expressions to be matched.