Add assertions checking SignExtend{32,64}'s bit width.

Summary:
The bit width must be greater than zero, otherwise we shift by the
integer's width, which is UB.  Also (more obviously) the width must be
less than or equal to the integer's width, otherwise we shift by a
negative number, which is also UB.

Reviewers: rnk

Subscribers: llvm-commits, dylanmckay

Differential Revision: https://reviews.llvm.org/D22442

llvm-svn: 275720