[PATCH] InstCombine: Narrow switch instruction operands using known bits

[Akira Hatanaka]

This patch adds an inst-combine optimization which narrows the operands of a switch instruction if the upper bits are known to be all 1 or 0.

For example, since the upper 32-bits are all zero in the following switch instruction,

%and = and i64 %a, 4294967295
switch i64 %and, label %sw.default [
  i64 10, label %return
  i64 100, label %sw.bb1
  i64 1001, label %sw.bb2
]

we can turn it into a 32-bit instruction:

switch i32 %and, label %sw.default [
  i32 10, label %return
  i32 100, label %sw.bb1
  i32 1001, label %sw.bb2
]

http://reviews.llvm.org/D5691

Files:
  lib/Transforms/InstCombine/InstructionCombining.cpp
  test/Transforms/InstCombine/narrow-switch.ll
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