RFR: 8221343: x86_32 crashes on startup with "_hwm out of range"

[Claes Redestad]

Hi,

some of the RegMasks allocated in Matcher::init_first_stack_mask aren't
properly initialized on 32-bit builds, which hits an assert after
JDK-8220159.  One simple fix is to explictly construct empty RegMasks
into the allocated memory.

Bug:    https://bugs.openjdk.java.net/browse/JDK-8221343
Webrev: http://cr.openjdk.java.net/~redestad/8221343/open.00/

Testing: verified reproducer in bug, pass tier1-3

Thanks!

/Claes