Teach instcombine to transform a bitcast/(zext|trunc)/bitcast sequence

Teach instcombine to transform a bitcast/(zext|trunc)/bitcast sequence
with a vector input and output into a shuffle vector.  This sort of
sequence happens when the input code stores with one type and reloads
with another type and then SROA promotes to i96 integers, which make
everyone sad.

This fixes rdar://7896024

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@103354 91177308-0d34-0410-b5e6-96231b3b80d8