1. Much of the lowering code for vector operations was not properly
checking that the input operand was in a register or memory. This
problem could be exhibited by passing undef values as inputs.

=> Change the vector legalization code to legalize input operands to
register or memory before producing instructions that use the
operands. Also, append a suffix to the variable names in the vector
legalization code to clarify the legalization status of the values.

2. Undef values should never be emitted directly. Rather, they should
have been appropriately legalized to a zero value.

=> To enforce this, make ConstantUndef::emit() issue an error
message. Do this in the x86 backend, as other backends may decide to
treat undef values differently.

3. The regalloc_evict_non_overlap test was loading from an undef
pointer. Subzero was not handling this correctly (the undef pointer was
being emitted without being legalized), but it does not have to handle
this case since PNaCl IR disallows undef pointers.

=> Fix the regalloc_evict_non_overlap test to use an inttoptr instead of
directly loading from the undef pointer. Also, add an assert in
IceTargetLoweringX8632::FormMemoryOperand() to make sure that undef
pointers are never encountered.

BUG=none
R=jvoung@chromium.org, stichnot@chromium.org

Review URL: https://codereview.chromium.org/432613002