[LVI] Relax the assertion about LVILatticeVal type in getConstantRange

The problem was triggered by my recent change in CVP (D23059). Current code expected that integer constants are represented by constantrange LVILatticeVal and never represented as LVILatticeVal with constant tag. That is true for ConstantInt constants, although ConstantExpr integer type constants are legally represented as constant LVILatticeVal.

This code fails with CVP change in:

@b = global i32 0, align 4
define void @test6(i32 %a) {
bb:
  %add = add i32 %a, ptrtoint (i32* @b to i32)
  ret void
}
Currently getConstantRange code is not executed by any of the upstream passes. I'm going to add a test case to test/Transforms/CorrelatedValuePropagation/add.ll once I resubmit the CVP change.

Reviewed By: sanjoy

Differential Revision: http://reviews.llvm.org/D23194

llvm-svn: 278217