[AArch64] Improve code generation for logical instructions taking

immediate operands.

This commit adds an AArch64 dag-combine that optimizes code generation
for logical instructions taking immediate operands. The optimization
uses demanded bits to change a logical instruction's immediate operand
so that the immediate can be folded into the immediate field of the
instruction.

This recommits r300913, which broke bots because I didn't fix a call to
ShrinkDemandedConstant in SIISelLowering.cpp after changing the APIs of
TargetLoweringOpt and TargetLowering.

rdar://problem/18231627

Differential Revision: https://reviews.llvm.org/D5591

llvm-svn: 300930