[CodeGen] Rename MachineInstr::isInvariantLoad to isDereferenceableInvariantLoad.  NFC

Summary:
I want to separate out the notions of invariance and dereferenceability
at the MI level, so that they correspond to the equivalent concepts at
the IR level.  (Currently an MI load is MI-invariant iff it's
IR-invariant and IR-dereferenceable.)

First step is renaming this function.

Reviewers: chandlerc

Subscribers: MatzeB, jfb, llvm-commits

Differential Revision: https://reviews.llvm.org/D23370

llvm-svn: 281125
diff --git a/llvm/lib/CodeGen/TargetInstrInfo.cpp b/llvm/lib/CodeGen/TargetInstrInfo.cpp
index 1652e48..1b62b2b 100644
--- a/llvm/lib/CodeGen/TargetInstrInfo.cpp
+++ b/llvm/lib/CodeGen/TargetInstrInfo.cpp
@@ -866,7 +866,7 @@
     return false;
 
   // Avoid instructions which load from potentially varying memory.
-  if (MI.mayLoad() && !MI.isInvariantLoad(AA))
+  if (MI.mayLoad() && !MI.isDereferenceableInvariantLoad(AA))
     return false;
 
   // If any of the registers accessed are non-constant, conservatively assume