s390x: Optimize CLC for 1, 2, 4, and 8 bytes

s390x: Optimize CLC for 1, 2, 4, and 8 bytes

The CLC instruction compares two memory areas with sizes from 1 up to 256
bytes.  Currently Valgrind always implements it with a bytewise loop.

Add special handling for the sizes 1, 2, 4, and 8.  Realize CLC with an
8-, 16-, 32-, and 64-bit integer comparison, respectively, in those cases.
Apart from a slight optimization this also improves the diagnostics for
uninitialized values since it avoids the manufactured conditional jump
that breaks out of the loop over the individual bytes.