Use deserialized constant pool for LdConst

[oxade]

Motivation

LdConst currently deserializes the BCS encoded constants on every invocation. This is unnecessary and also makes it considerably slower than Ld* of same data size. This PR uses a pool of pre-deserialized constants to avoid this.

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

Standard tests Test failure due to ValueImpl not impl Send