[InstCombine] Match scalable splats in m_ImmConstant (#132522)

#118806 fixed an infinite loop in FoldShiftByConstant that could occur
when the shift amount was a ConstantExpr.

However this meant that FoldShiftByConstant no longer kicked in for
scalable vectors because scalable splats are represented by
ConstantExprs.

This fixes it by allowing scalable splats of non-ConstantExprs in
m_ImmConstant, which also fixes a few other test cases where scalable
splats were being missed.

But I'm also hoping that UseConstantIntForScalableSplat will eventually
remove the need for this.

I noticed this when trying to reverse a combine on RISC-V in #132245,
and saw that the resulting vector and scalar forms were different.

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Co-authored-by: Yingwei Zheng <dtcxzyw@qq.com>

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