This patch moves fixing up the incoming value of phis for
VPWidenIntOrFpInductionRecipes to after the VPlan is executed.
At the moment, the code in widenIntOrFpInduction relies on the vector
loop latch being created *before* the VPlan is executed. This prevents
us from modeling the vector latch, preheader and exit blocks in VPlan.
By patching up the incoming values for induction PHIs after VPlan
execution, we remove the need to create the latch up front. This enables
generating the vector latch directly from VPlan, which in turn is
required to enable modeling prehader and exit blocks.
The patch uses a workaround to access both the induction increment and
the vector phi for the recipe. It adds 2 new VPValues which are set to
them. While this is a temporary workaround, this will be removed once
the induction code generation is moved to work on VPlan more directly.
See D111303 for the next step, making the induction increment explicit
as VPValue operand.