[InstCombine] Extend pattern to replace shuffle's insertelement operand

In D71220 a pattern was added to replace shuffle's insertelement operand
if inserted scalar is not demanded. The pattern was added only for
the case where the shuffle's mask size is equal to element's vector size.
However, that condition is not required because the pattern does not
change the shuffle vector size.

This patch extends the pattern to also include cases where shuffle's mask
size is not equal to element's vector size.

Differential Revision: https://reviews.llvm.org/D112318