[FastISel] Don't trivially kill extractvalues (PR49467)

All extractvalues of the same value at the same index will map to
the same register, so even if one specific extractvalue only has
one use, we should not mark it as a trivial kill, as there may be
more extractvalues later.

Fixes https://bugs.llvm.org/show_bug.cgi?id=49467.

Differential Revision: https://reviews.llvm.org/D98145