[Attributes] Fix a bug in AttributeList::get so it can handle a mix of FunctionIndex...

[Attributes] Fix a bug in AttributeList::get so it can handle a mix of FunctionIndex and ReturnIndex/arg indices at the same time

The code uses the index of the last element in the sorted array to determine the maximum size needed for the vector. But if the last index is a FunctionIndex(~0), attrIdxToArrayIdx will return 0 and the vector will have size 1. If there are any indices before FunctionIndex, those values would return a value larger than 0 from attrIdxToArrayIdx. So in this case we need to look in front of the FunctionIndex to get the true size needed.

Differential Revision: https://reviews.llvm.org/D45632

llvm-svn: 330136