[IR] Allow constant folding (insertelement <vscale x 2 x i32> zeroinitializer, i32 0, i32 i32 0.

Most of insertelement constant folding is blocked if the vector type
is scalable. I believe we can make an exception for inserting null
into an all zeros vector.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D123413