Tweak IRBuilder to explicitly call the setName(const char*) method on Value

Tweak IRBuilder to explicitly call the setName(const char*) method on Value
instead of passing the name into the instruction ctors.  Since most
instruction ctors take their name as an std::string, this avoids copying the
string to the heap and a malloc and free.

Patch by Pratik Solanki!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@52832 91177308-0d34-0410-b5e6-96231b3b80d8