Factorize common code out of the InstructionSimplify shift logic. Add in

threading of shifts over selects and phis while there.  This fires here and
there in the testsuite, to not much effect.  For example when compiling spirit
it fires 5 times, during early-cse, resulting in 6 more cse simplifications,
and 3 more terminators being folded by jump threading, but the final bitcode
doesn't change in any interesting way: other optimizations would have caught
the opportunity anyway, only later.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@123441 91177308-0d34-0410-b5e6-96231b3b80d8

test/Transforms/InstSimplify/2011-01-14-Thread.ll

@@ -0,0 +1,9 @@
+; RUN: opt < %s -instsimplify -S | FileCheck %s
+
+define i32 @shift_select(i1 %cond) {
+; CHECK: @shift_select
+  %s = select i1 %cond, i32 0, i32 1
+  %r = lshr i32 %s, 1
+  ret i32 %r
+; CHECK: ret i32 0
+}