Remove unwanted side effects in `SPLIT_EQ_STRIP_L` (#6689)

Remove unwanted side effects in `SPLIT_EQ_STRIP_L` (#6689)

Fixes #6681. When checking whether SPLIT_EQ_STRIP_L applies, we were
using sripSymbolicLength, which changes its inputs depending on how
many elements of the concatenation it could strip. However, one of the
arguments, pfxv0, was reused across multiple checks if the strip did
not result in a rewrite. Later invocations were wrong as a result. This
commit changes the call to stripSymbolicLength() to use a new copy of
the vector instead.