AMDGPU: Avoid emitting "true" predicates

Empty condition strings are considerde always true. This removes a lot
of clutter from the generated matcher tables.

This shrinks the source size of AMDGPUGenDAGISel.inc from 7.3M to
6.1M.

llvm-svn: 367326
diff --git a/llvm/lib/Target/AMDGPU/AMDGPUInstructions.td b/llvm/lib/Target/AMDGPU/AMDGPUInstructions.td
index 6efd578..885bfc7 100644
--- a/llvm/lib/Target/AMDGPU/AMDGPUInstructions.td
+++ b/llvm/lib/Target/AMDGPU/AMDGPUInstructions.td
@@ -75,7 +75,7 @@
      let isCodeGenOnly = 1;
 }
 
-def TruePredicate : Predicate<"true">;
+def TruePredicate : Predicate<"">;
 
 class PredicateControl {
   Predicate SubtargetPredicate = TruePredicate;