Subzero: Use C++11 member initializers where practical.

Also change the pattern "foo() {}" into "foo() = default;" for ctors and dtors.

Generally avoids initializing unique_ptr<> members to nullptr in a .h file, because that requires knowing the definition of the underlying class which may not be available to all includers.

BUG= none
R=jpp@chromium.org

Review URL: https://codereview.chromium.org/1197223002
diff --git a/src/IceTargetLoweringMIPS32.cpp b/src/IceTargetLoweringMIPS32.cpp
index 3655cb7..2cf5953 100644
--- a/src/IceTargetLoweringMIPS32.cpp
+++ b/src/IceTargetLoweringMIPS32.cpp
@@ -30,8 +30,7 @@
 
 namespace Ice {
 
-TargetMIPS32::TargetMIPS32(Cfg *Func)
-    : TargetLowering(Func), UsesFramePointer(false) {
+TargetMIPS32::TargetMIPS32(Cfg *Func) : TargetLowering(Func) {
   // TODO: Don't initialize IntegerRegisters and friends every time.
   // Instead, initialize in some sort of static initializer for the
   // class.