[AMDGPU] Aggressively fold immediates in SIFoldOperands

Previously SIFoldOperands::foldInstOperand would only fold a
non-inlinable immediate into a single user, so as not to increase code
size by adding the same 32-bit literal operand to many instructions.

This patch removes that restriction, so that a non-inlinable immediate
will be folded into any number of users. The rationale is:
- It reduces the number of registers used for holding constant values,
  which might increase occupancy. (On the other hand, many of these
  registers are SGPRs which no longer affect occupancy on GFX10+.)
- It reduces ALU stalls between the instruction that loads a constant
  into a register, and the instruction that uses it.
- The above benefits are expected to outweigh any increase in code size.

Differential Revision: https://reviews.llvm.org/D114643