Fix the display of pipeline values in debug prints.
Fix multi-register reads.

he logic introduced in [dbbe85901fd1b505676f719c7d876c5fa126a8f2] to
read zero instead of empty on local registers introduced a bug. If an
instruction reads multiple registers at once (eg store double on
MT-SPARC), and only the 2nd register is empty, the "read zero instead"
would erase the value already partially read from the 1st register as
well. This patch ensures that the test is made before the previous
value is read back.
Fix the MT-SPARC test suite.