Fix constant-propagation test pattern

With #7331 in combination with earlier changes to the simplifier the
left-hand side instance to check for now has a different SSA index.
As #7331 was not rebased on top of the simplifier changes, this went
unnoticed.

Author: tautschnig

regression/cbmc-library/memcpy-01/constant-propagation.desc

@@ -1,7 +1,7 @@
 CORE
 main.c
 --show-vcc
-main::1::m!0@1#1 = .*byte_extract_(big|little)_endian\(cast\(44, signedbv\[\d+\]\*\), 0, signedbv\[\d+\]\).*
+main::1::m!0@1#2 = .*byte_extract_(big|little)_endian\(cast\(44, signedbv\[\d+\]\*\), 0, signedbv\[\d+\]\).*
 ^EXIT=0$
 ^SIGNAL=0$
 --