Trying to shift by a ConstantRelocatable causes an assertion failure in the integrated assembler, because the shift value must be Imm8.  Maybe this is possible to express via ELF relocations, but it doesn't seem worth it.  Especially since ConstantRelocatables refer to addresses and it doesn't make sense to shift by an address.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4256
R=jvoung@chromium.org, kschimpf@google.com

Review URL: https://codereview.chromium.org/1262003003.