Reserve G1 for frame offset stuff and use it to handle large stack frames.

For example, instead of emitting this:

test:
        save -40112, %o6, %o6   ;; imm too large
        add %i6, -40016, %o0    ;; imm too large
        call caller
        nop
        restore %g0, %g0, %g0
        retl
        nop

emit this:

test:
        sethi 4194264, %g1
        or %g1, 848, %g1
        save %o6, %g1, %o6
        sethi 4194264, %g1
        add %g1, %i6, %g1
        add %i1, 944, %o0
        call caller
        nop
        restore %g0, %g0, %g0
        retl
        nop

which doesn't cause the assembler to barf.

llvm-svn: 24880