[llvm] [GlobalISel] Avoid double DenseMap lookups in ValueToVRegInfo (NFC) (PR #221741)

[Cullen Rhodes via llvm-commits]

c-rhodes wrote:

small compile-time improvements aarch64-O0-g: https://llvm-compile-time-tracker.com/compare.php?from=97cbc1e404b980edc58bfbcabb6f1c61793b624b&to=fbf62152c51f2327cb2f4a382df90b3f345a8866&stat=instructions:u

https://github.com/llvm/llvm-project/pull/221741