This changes the return type of
  - StringBuiltinsAssembler::LoadSurrogatePairAt
  - CodeStubAssembler::StringCharCodeAt
from TNode<Uint32T> to TNode<Int32T>.

This is justified because both functions only
return values in the positive range of signed
integer. This improves interoperatability, as
Int32T can be SmiTagged, while this is not
allowed for Uint32T.

Bug: v8:7270

Change-Id: I2768b6ec320fa0fbcf3e55af784339472fa4909e
Reviewed-on: https://chromium-review.googlesource.com/861782Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50542}