[X86] Freeze vXi8 shl(x,1) -> add(x,x) vector fold (PR50468)

We don't have any vXi8 shift instructions (other than on XOP which is handled separately), so replace the shl(x,1) -> add(x,x) fold with shl(x,1) -> add(freeze(x),freeze(x)) to avoid the undef issues identified in PR50468.

Split off from D106675 as I'm still looking at whether we can fix the vXi16/i32/i64 issues with the D106679 alternative.

Differential Revision: https://reviews.llvm.org/D108139