[LVI] Fix LVI compile time regression around constantFoldUser()

Summary:
Avoid checking each operand and calling getValueFromCondition() before calling
constantFoldUser() when the instruction type isn't supported by
constantFoldUser().

This fixes a large compile time regression in an internal build.

Reviewers: sanjoy

Reviewed By: sanjoy

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D36552

llvm-svn: 310545