[CostModel] Use cost of target trunc type when only it is the only use of a non-regis...

[CostModel] Use cost of target trunc type when only it is the only use of a non-register sized load

The code size cost model for most targets uses the legalization cost for the type of the pointer of a load. If this load is followed directly by a trunc instruction, and is the only use of the result of the load, only one instruction is generated in the target assembly language. This adds a check for this case, and uses the target type of the trunc instruction if so.

This did not show any changes in CTMark code size benchmarks.

Reviewers: paquette, samparker, dmgreen

Differential Revision: https://reviews.llvm.org/D109388