mips32/mips64: Fix compiler warnings.

Fix some compiler warnings when compiling Valgrind for no mips arch.


git-svn-id: svn://svn.valgrind.org/vex/trunk@2741 8f6e269a-dfd6-0310-a8e1-e2731360e62c
Eliminate IRExprP__VECRET and IRExprP__BBPTR and introduce two new
IRExpr kinds instead: Iex_VECRET and Iex_BBPTR. Add constructor 
functions and adjust ppIRExpr, typeOfIRxpr and deepCopyExpr. The
rest is mechanics.


git-svn-id: svn://svn.valgrind.org/vex/trunk@2742 8f6e269a-dfd6-0310-a8e1-e2731360e62c