[Jump Threading] Unfold a select insn that feeds a switch via a phi node

Currently when a select has a constant value in one branch and the select feeds
a conditional branch (via a compare/ phi and compare) we unfold the select 
statement. This results in threading the conditional branch later on. Similar
opportunity exists when a select (with a constant in one branch) feeds a 
switch (via a phi node). The patch unfolds select under this condition. 
A testcase is provided.

llvm-svn: 350931