[InstCombine] Update transformations to use poison for insertelement/shufflevector's placeholder value (1/2)

[aqjune]

Summary

This patch updates InstCombine transformations except InstCombineVectorOps.cpp to create
insertelement/shufflevector with poison placeholder.

The updates in InstCombineVectorOps.cpp will be done in the next patch.

It is close to the end..!

[aqjune]

llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp

1728

Guarded by this branch condition, the for loop at line 1740 never adds a mask that makes Shuffle point to the second operand vector (which was previously Undef, but now Poison).

llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp

2613

This creates insertelement <1 x X86_mmx> undef, Elem, 0
Replacing undef with poison has no visible effect.

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

1693

Be conservative, and create shufflevector with undef if Mask may point to the second vector