The tricky part here is to take away one register from register
allocation for the mask. The only problem is with calls that need
an input operand to be passed in the poison register. For such calls,
we change the register constraint in the instruction selector
to pass the value in whatever place the register allocator sees fit.
During code generation, we then copy the value from that place
to the poison register. By that time, the mask is not necessary
(once we bake the mask into the target, it should be done before
this move).

For the branches, the mask update does not use cmov (unlike x64)
because cmov does not take an immediate and we do not have
a scratch register. Instead we use bit-twiddling tricks
(suggested by @tebbi). For example, here is the code for masking
register update after a bailout on non-zero:

  jnz deopt_bailout    ;; Bailout branch
  setnz bl             ;; These three instructions update the mask
  add  ebx, 255
  sar  ebx, 31

(On x64, the sequence is:

  jnz deopt_bailout
  mov r10, 0      ;; We have a scratch register for zero
  cmovnz r9, r10  ;; Set to zero if we execute this branch
                  ;; in branch mis-speculation
)


This CL also fixes a bug in register configuration, where we used
to wrongly restrict the array of register name.

Change-Id: I5fceff2faf8bdc527d9934afc284b749574ab69e
Bug: chromium:798964
Reviewed-on: https://chromium-review.googlesource.com/946251
Commit-Queue: Jaroslav Sevcik <jarin@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51798}