Fix an issue with "Rearrange MIPS INSN* masks" patch.

This fixes an issue with Mark Shinwell's "Rearrange MIPS INSN* masks" patch
(https://sourceware.org/ml/binutils/2007-11/msg00231.html).  In the patch
the pref instruction had its membership flags changed from I4|I32|G3 to
I4_32|G3.  Unfortunately G3 was defined as being I4, which made the actual
expanded flags as: I4|I32|I4 and therefore the membership flags should have
been I4_32.  Since the patch was committed G3 was redefined to be I4|EE.  This
fix just removes I4 from G3 making the expanded membership flags for pref as
I4_32|EE.

ChangeLog:

opcodes/
	* mips-opc.c (G3): Remove I4.