[APInt] Rename getSignBit to getSignMask

[APInt] Rename getSignBit to getSignMask

getSignBit is a static function that creates an APInt with only the sign bit set. getSignMask seems like a better name to convey its functionality. In fact several places use it and then store in an APInt named SignMask.

Differential Revision: https://reviews.llvm.org/D32108

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@300856 91177308-0d34-0410-b5e6-96231b3b80d8