[GlobalISel][IRTranslator] Fix <1 x Ty> handling in ConstantExprs

Summary:
ConstantExprs involving operations on <1 x Ty> could translate into MIR
that failed to verify with:
*** Bad machine code: Reading virtual register without a def ***

The problem was that translate(const Constant &C, Register Reg) had
recursive calls that passed the same Reg in for the translation of a
subexpression, but without updating VMap for the subexpression first as
translate(const Constant &C, Register Reg) expects.

Fix this by using the same translateCopy helper function that we use for
translating Instructions. In some cases this causes extra G_COPY
MIR instructions to be generated.

Fixes https://bugs.llvm.org/show_bug.cgi?id=45576

Reviewers: arsenm, volkan, t.p.northover, aditya_nandakumar

Subscribers: jvesely, wdng, nhaehnle, rovka, hiraditya, kerbowa, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78378