A previous CL removed the kNoThrow flags from both
SpeculativeBigIntAdd and SpeculativeBigIntSubtract. This introduced a
bug, because the JSTypeHintLowering phase, where these operators are
introduced during inlining, does not support the generation of throwing
operators.

Since these operators always deoptimize in case of an error, instead of
throwing the exception directly, it is safe to mark them as kNoThrow.

Bug: chromium:1091461
No-Try: true
No-Tree-Checks: true
Change-Id: I551616b0c462647574e5af8824d9ed7b3252659d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2235113
Commit-Queue: Nico Hartmann <nicohartmann@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68254}