[AMDGPU] Fixed constexpr expansion to handle multiple uses

Recently added convertConstantExprsToInstructions() does not handle
a case when a same ConstantExpr used multiple times in the same
instruction. A first use is replaced and the rest of the uses in the
instruction are replaced as well with the replaceUsesOfWith(). Then
function attempts to replace a constant already destroyed.

So far this interface is only used by the AMDGPU BE.

Differential Revision: https://reviews.llvm.org/D104425