JIT: Fix for 2813841, use core regs for sub-word data

In an attempt to avoid unnecessary register copies, the JIT allows
data items to live in either floating point or core registers until
an instruction is used which requires one or the other.  The bug here
was that sub-word data was allowed to live in floating point registers
at the point of a load or store.  This cl forces the use of core registers
in those cases.

Change-Id: Iaee57545c6a62990186a5d0ab5bb22728d75dd60