AMDGPU: Fix crash when folding immediates into multiple uses

Summary:
When an immediate is folded by constant folding, we re-scan the entire
use list for two reasons:

1. The constant folding may have created a new use of the same reg.
2. The constant folding may have removed an additional use in the list
   we're currently traversing (e.g., constant folding an S_ADD_I32 c, c).

However, this could previously lead to a crash when an unrelated use was
added twice into the FoldList. Since we re-scan the whole list anyway, we
might as well just clear the FoldList again before we do so.

Using a MIR test to show this because real code seems to trigger the issue
only in connection with some really subtle control flow structures.

Fixes GL45-CTS.shading_language_420pack.binding_images on gfx9.

Reviewers: arsenm

Subscribers: kzhuravl, wdng, yaxunl, dstuttard, tpr, llvm-commits, t-tye

Differential Revision: https://reviews.llvm.org/D35416

llvm-svn: 308314