[AMDGPU] Fix scalar operand folding bug that causes SHOC performance regression.

Detailed description: SIFoldOperands::foldInstOperand iterates over the
operand uses calling the function that changes def-use iteratorson the
way. As a result loop exits immediately when def-use iterator is
changed. Hence, the operand is folded to the very first use instruction
only. This makes VGPR live along the whole basic block and increases
register pressure significantly. The performance drop observed in SHOC
DeviceMemory test is caused by this bug.

Proposed fix: collect uses to separate container for further processing
in another loop.

Testing: make check-llvm
SHOC performance test.

Reviewers: rampitec, ronlieb

Differential Revision: https://reviews.llvm.org/D56161

llvm-svn: 350350