Clarify wording in the LangRef around !invariant.load

Clarify the wording around !invariant.load to properly reflect the semantics of such loads with respect to control dependence and location lifetime.  To the best of my knowledge, the revised wording respects the actual implementation and understanding of issues involved highlighted in the recent 'Optimization hints for "constant" loads' thread on LLVMDev.  

In particular, I'm aiming for the following results:
- To clarify that an invariant.load can fault and must respect control dependence.  In particular, it is not sound to unconditionally pull an invariant load out of a loop if that loop would potentially never execute.  
- To clarify that the invariant nature of a given pointer does not preclude the modification of that location through a pointer which is unrelated to the load operand.  In particular, initializing a location and then passing a pointer through an opaque intrinsic which produces a new unrelated pointer, should behave as expected provided that the intrinsic is memory dependent on the initializing store.  
- To clarify that storing a value to an invariant location is defined.  It can not, for example, be considered unreachable.  The value stored can be assumed to be equal to the value of any previous (or following!) invariant load, but the store itself is defined.  

I recommend that anyone interested in using !invariant.load, or optimizing for them, read over the discussion in the review thread.  A number of motivating examples are discussed.

Differential Revision: http://reviews.llvm.org/D6346



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222700 91177308-0d34-0410-b5e6-96231b3b80d8
Convert test to FileCheck and use CHECK-LABEL

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222704 91177308-0d34-0410-b5e6-96231b3b80d8
Bug 21610: Canonicalize min/max fcmp selects to use ordered comparisons

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222705 91177308-0d34-0410-b5e6-96231b3b80d8
Incorporate review comments from r221742

This change implements the comment and style changes Sean requested during post commit review with r221742.  Sorry for the delay.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222707 91177308-0d34-0410-b5e6-96231b3b80d8